The objective of the present work was to study effects of estradiol and progesterone on enzymatic, oxidative, and phagocytic function of human neutrophils. It was shown that sex hormones applied at the doses equivalent to their concentrations in pregnant women selectively stimulated spontaneous activity of neutrophilic elastase and increased myeloperoxidase enzymatic activity. Neither estradiol nor progesterone had apparent effect on baseline spontaneous hemiluminescence of neutrophils but stimulated E.coli-induced luminol-dependent hemiluminescence. Simultaneously, both steroids inhibited phagocytic activity of these cells. It is concluded that this modulating action of female sex steroids on neutrophil functions is likely to come out during pregnancy.