HbCO in blood sampled from 20 mothers and newborns immediately after birth was measured with a new, simple gas-chromatographic method for CO. The mean ratio of HbCO to total hemoglobin for 13 non-smoking mothers did not differ significantly from that for their infants (mean 0.38%, SD 0.26% vs 0.38%, SD 0.13%), but the HbCO ratio varied more in the mothers than in the infants (P less than 0.05). The infants of seven cigarette-smoking mothers, tobacco-abstinent for 7 h during labor, had higher HbCO ratios than their mothers (mean 1.88% vs 1.28%, P less than 0.05). The results are in harmony with the concept of equal affinities of fetal and adult hemoglobin for CO and a long time for passage of CO across the placenta.