Neutrophil hexose monophosphate (H.M.P.) shunt activity was measured by the conversion of 14C-1-glucose to 14CO2 during ingestion of polystyrene latex particles. Unconjugated bilrubin at the concentration of 1 X 10(5) M was found to markedly inhibit H.M.P. shunt activity. Since H.M.P. shunt activity of neutrophils is an important prerequisite of microbicidal function, it is suggested that jaundiced newborns may be more susceptible to bacterial infections.