Patient informed consent (short version, complete version was approved by ethics committee and is enclosed in appendix): Study description: Each surgical procedure has its own risk so called postoperative infectious complications. One possibility how to prevent those complications is the preventive administration of antibiotics just prior the procedure - so called antibiotic (ATB) prophylaxis. There is no clear evidence for positive effect of ATB prophylactic use in some acute surgical procedures. On the other hand introduction of ATB prophylaxis has its complications. Aim of study: Aim of the study is the recommendation whether in given cases administration of ATB prophylaxis has positive effect on reduction of incidence of postoperative infectious complications and whether this benefit is higher than risks related to ATB administration. Inclusion in study and conductance: Single inclusion is only based on the decision of patient (approximately 500 patients). Study participation will be offered to patients who will be operated on due to acute appendicitis, perforated stomach or duodenal ulcer or due to ileus of small bowel. Pregnant and lactating females are excluded from the study. After patient's consent with study participation the intravenous preparation will be administered which contains active substance (antibiotic) or placebo (only saline solution). Both administered substances are registered preparations administered for given purpose in general praxis. The chance of application of either active preparation and or placebo is 50:50. Patient will be telephonically inquired about concrete postoperative complications on day 30 and 60 after surgery. If there is suspicion on complication, patient will be examined and treated using standard procedure. He will be further asked for completion of questionnaire about life quality. Health risks: This study is not related to practically any excessive risk. ATB and placebo administration does not in any way influence the surgical procedure or postoperative care. Non-invasive sample collection for cultivation (urine collection, sputum collection -secretion from respiratory tract mucosa, smear from throat and stool). Compensation for bodily harm: If patient obliges to study physician's instructions and her/his team and health damage of patient occurs as a result of administration of any preparation, the study sponsor will cover all cost for treatment of this damage which is not covered by patient's health insurance. Patient's declaration: I, undersigned, agree with my participation in study. I am older than 18 years of age. I was informed in detail about the aim of study, its procedures and what it is expected from me. Study physician explained me the expected benefits and possible health risks. I acknowledge that conducted study is an investigation activity. Study is randomized and therefore I acknowledge possibility of random assignment into individual groups which differ in therapy. I fully understood and I have the opportunity to ask additional questions which were satisfactory answered to me. I informed study conducting physician about all my diseases and medications which I have been taking over the last 28 days and also about medication which I currently take. I declare that I do know that I am pregnant at time of inclusion in the study. I understood that I can discontinue or withdraw my participation in the study at any time without affecting course of my further treatment. My personal data will be kept in full confidentiality after study enrollment according to applicable laws in the Czech Republic. Based on my given consent my original medical information could be accessed to authorized acquired data by sponsor representatives, independent ethics committees and international or local competent authorities (The State Institute for Drug Control in the Czech Republic). Confidentiality protection of personal data is guaranteed for those cases. There will not be any cost to me for my participation in study and concomitantly study enrollment is not related to any financial reward. I understood that my name will never be present in reports about this study. (date, name, signature) Cooperation Working site of co-worker participates in postgraduate education; co-worker is recognized surgical expert and therefore he is guarantee for correct study conductance and results applications in praxis; the cooperation will be confirmed by agreement.