Participants receive a comprehensive psychiatric, neurological and physical evaluation. Children will initially receive penicillin tablets, and then will be randomly assigned to receive either penicillin or placebo tablets for 6 months. Children will be monitored monthly by either in-person visits or a telephone interview. Any child who has a significant increase in his or her OCD or tics is taken off the randomized medication and put on open-label penicillin for the rest of the study.