Cytologic and bacteriologic responses, and changes in cytokine activity were evaluated in secretions of Staphylococcus aureus-infected mammary glands after treatment of heifers with recombinant bovine interferon gamma (rbIFN gamma) or interleukin 2 (rbIL-2). Two groups of 4 heifers each, experimentally infected with 10(7) colony-forming units (CFU) of S aureus, were injected in 2 quarters via the teat canal, with 10(5) U of rbIFN gamma (trial 1) or 7.5 X 10(5) U of rbIL-2 (trial 2) 2 weeks after experimentally induced infection; control quarters received phosphate-buffered saline solution. Mammary secretion samples were taken on days 0, 1, 2, 3, 4, 7, and 14 after cytokine infusion. Secretions were diluted 1:10 and used to perform somatic cell counts (SCC), differential cell counts, and CFU enumerations, and to determine the number of leukocytes expressing major histocompatibility complex class-II (MHC II) antigens. In addition, mammary secretion samples taken on days 0, 1, and 2 were processed to obtain skimmed milk for evaluation of rbIFN gamma- and rbIL-2-like activities. Treatment with rbIFN gamma did not influence SCC, or differential or bacteria counts, or the number of leukocytes expressing MHC II antigens. However, rbIL-2 stimulated leukocytosis, which may have reduced bacteria counts early in the trial; treatment with this cytokine also increased the neutrophil, macrophage, lymphocyte, and eosinophil counts in secretions. Similarly, numbers of MHC II-positive leukocytes were greater in rbIL-2-treated quarters vs controls. Compared with day 0, IFN gamma-like activity was increased on only day 1 in both trials. Interleukin-2-like activity was not influenced in the rbIFN gamma trial, but was increased on days 1 and 2 in the rbIL-2 trials. Results indicated that neither cytokine may have had a major influence on the course of established S aureus infections. However, the increased SCC in rbIL-2-treated quarters may have accounted for the reduction in CFU throughout the trial after treatment with this cytokine. Greatest cytokine-like activity was observed on day 1; however, the consequences of cytokine activity, such as the sustained eosinophilia after rbIL-2 treatment, were detected over the 14-day trial period, indicating possible prolonged action.