The primary objective is to describe the levels of mercury in the blood of premature newborns greater than or equal to 32 weeks and less than 37 weeks gestation and birth weight greater than or equal to 2000 grams but less than 3000 grams, who receive the routine birth dose of Hepatitis B immunization containing ethyl mercury in the form of thimerosal and the birth dose of BCG immunization, and to evaluate the excretion of mercury in these newborns by examining mercury levels in stool and urine. Secondary objectives are: 1) to determine the half-time of mercury in blood following vaccination of premature and low birth weight infants with thimerosal-containing vaccines, and 2) to screen premature and low birth weight infants for evidence of early effects of thimerosal on the kidney by measurement of urinary gamma glutamyl transpeptidase levels. All infants will have stool, urine, and blood samples collected prior to receipt of routine birth hepatitis B and BCG immunization. Cord blood will serve as the baseline blood specimen. Each infant will be seen once at a predetermined follow-up visit time point. There will be 6 follow-up visit time points, with sampling from 12 infants at each time point. All infants will provide blood, urine, and stool specimens at the assigned follow-up visit.