The experimentation covered three general areas relating to the effects of steroid hormones on bovine granulocyte function: (1) characterization and comparison of neutrophil and eosinophil function in cattle, (2) the effects of glucocorticoids on bovine neutrophil function, and (3) the effects of estradiol and progesterone on bovine neutrophil function;A method was developed for separating eosinophils and neutrophils from the peripheral blood of normal cattle using centrifugation through 1.135 specific gravity Ficoll-Hypaque. Neutrophils were significantly more active than eosinophils in the ingestion of Staphylococcus aureus, and in antibody-dependent cell-mediated cytotoxicity (ADCC). Eosinophils were much more active in the resting and stimulated iodination test and they contained higher levels of peroxidase. The two cell types displayed equivalent nitroblue tetrazolium (NBT) reduction and random migration under agarose;The administration of a single 40 mg dose of dexamethasone to cattle caused enhanced random migration under agarose but impaired ingestion of S. aureus, NBT reduction, chemiluminescence, iodination, and ADCC by neutrophils. The repeated administration of 200 I.U. of adrenocorticotropic hormone to cattle produced increased serum cortisol levels, enhanced random migration by neutrophils, depressed iodination by neutrophils, and impaired lymphocyte blastogenesis. Increased cortisol had no effect on S. aureus ingestion, NBT reduction, or ADCC by neutrophils;The repeated administration of high dosages of estradiol to steers produced no significant effect on the total or differential white blood cell count, serum cortisol concentration, or neutrophil function. High dosages of progesterone in steers had no significant effect on the total or differential white blood cell count or serum cortisol concentration but it did cause enhanced random migration and impaired iodination by neutrophils. There was no significant effect on the ability of neutrophils to ingest S. aureus, reduce NBT, or mediate ADCC;Elevated serum estradiol during the estrous cycle of cows was associated with enhanced random migration and elevated serum cortisol concentration; there was no measurable effect on any of the other neutrophil function parameters evaluated. The enhancement of random migration may have been due to the concurrent elevation of serum cortisol concentration. Elevated serum progesterone during the estrous cycle was associated with enhanced random migration under agarose and ability to mediate ADCC, impairment of NBT reduction and iodination, and normal S. aureus ingestion by neutrophils.