Pregnancy has been associated with alterations of polymorphonuclear neutrophil (PMN) function. Superoxide anion production was studied in pregnant women paired with nonpregnant women of childbearing age. There was a significant decrease in the amount of cytochrome c reduced in response to 1µM N-formylmethionylleucylphenylalanine (fMet-Leu-Phe) but not to phorbol myristate acetate, 4 or 20 ng/ml. Chemotaxis was also depressed. Binding of tritiated fMet-Leu-Phe to PMNs from pregnant women was not defective. Incubation of normal cells in up to 10⁻⁶M estradiol or progesterone did not mimic the defect, but 10⁻⁷M progesterone caused a decrease in chemotaxis. Serum pooled from women with the defect had no effect on superoxide anion production by normal PMNs. PMN rosetting with IgG-sensitized human erythrocytes was normal. Defective production of superoxide anion may contribute to the amelioration of connective tissue disease and increased susceptibility to infection often seen during pregnancy.