Enzymatic peroxidase oxidation was used to study albumin-bilirubin binding in the serum of pregnant women, their term infants, and nonpregnant women. When the bilirubin titration curves were corrected for differing albumin concentrations in the three groups, the bilirubin binding of pregnant women was inferior to that of control women but better than that of their infants. Diminished maternal binding could not be explained by altered affinity. This suggests that the function of albumin during pregnancy is impaired by a factor common to both fetal and maternal circulations that appears to act by decreasing the number of available bilirubin binding sites.