Inhibition of serum bactericidal activity by bilirubin.

The results presented show that bilirubin inhibited the bactericidal activity of human serum. Addition of serum albumin (which tightly binds bilirubin) to the bactericidal assay prevented the inhibition mediated by bilirubin. These findings suggest that the decreased bactericidal activity of sera from neonates with hyperbilirubinemia may be due to the inhibition of the bactericidal activity by unconjugated bilirubin.