The goal of the study is to find out how much mercury is in the blood, urine, and stools of premature and low birth weight infants who have received standard childhood vaccines that are given to all infants in Argentina. Seventy-two newborn premature and low birth weight infants from Durand Acute General hospital will participate in this study. Each infant in the study will need to make two visits, one at the time of vaccination and another 12 hours to 30 days later. Urine, stool, and a small sample of blood will be taken at each visit.