Bilirubin production, as indexed by serum carboxyhemoglobin (HbCOc), was studied in a group of normal term Japanese infants and Caucasian controls during the second to third day of life. Stringent entry criteria were employed in order to eliminate infants with hemolysis or other known causes of increased bilirubin production. The mean HbCOcof the Japanese infants (0.69 ± 0.15% sat) was significantly higher than that of the Caucasian infants (0.58 ± 0.17% sat). The serum total bilirubin was also significantly higher in Japanese infants (11.1 ± 3.0 mg/dl versus 8.0 ± 2.2 mg/dl). This difference may be attributable to environmental and/or genetic factors.