Intravenous immune globulin (750 mg/kg), or an equivalent volume of a placebo, was administered during the first week of life, in a randomized trial, to 20 preterm neonates weighing 710 to 1800 g. A variety of laboratory and clinical values were measured serially and analyzed for possible untoward effects. Serum IgG levels were also determined serially. No differences in heart rate, respiratory rate, urine output, blood glucose, serum osmolality, BUN, SGPT, pH, blood gasses, serum electrolytes, total or direct bilirubin, blood leukocyte concentration, absolute neutrophil count, or blood platelet concentration were observed between the intravenous immune globulin (IVIG) and placebo recipients before or following IVIG administration. The red blood cell concentration of IVIG recipients diminished transiently and only slightly (P less than .05). Serum IgG levels increased from 503 +/- 162 mg/dL (X +/- SD) to 1492 +/- 201 mg/dL 15 minutes after the IVIG administration (P less than .001). After 8 days, serum IgG levels were still elevated, at 675 +/- 297 mg/dL. All patients randomized to receive the placebo experienced a diminution in serum IgG over this 8-day period (P less than .01). All 20 patients survived and none in either group had a documented nosocomial infection. This study suggests that IVIG can safely be administered to preterm neonates, resulting in serum IgG levels comparable to those of term infants.