This study was undertaken after the observation in a premature infant of a hemolytic anemia with Heinz bodies that appeared to result from administration of a multivitamin preparation. In vitro incubation of erythrocytes of premature infants with sodium ascorbate (0.1 mg/ml) for 3 hours significantly raised the number of Heinz body-containing cells from 17.6±5.7% to 27.2±8.2% (mean±SE). Erythrocytes of term infants and those of adults developed Heinz bodies after exposure to higher sodium ascorbate concentrations (1.0 mg/ml). Erythrocytes of adult and newborn guinea pigs were similarly affected by sodium ascorbate. Daily intraperitoneal injections of 500 mg of sodium ascorbate, given for 7 days to four adult guinea pigs, caused significant Heinz body formation. These studies indicate that the erythrocytes of premature infants are uniquely sensitive to the development of Heinz bodies after exposure to sodium ascorbate. The levels required to produce Heinz bodies in vitro are in the range of those found in vivo after routine administration of vitamin C to premature infants. The significance of these observations in the development of hyperbillrubinemia in premature infants and in the safety of vitamin C remains to be determined.