The production of interferon-gamma (IFN-gamma) in bovine peripheral blood mononuclear cells (PBMC) during the perinatal period was studied. IFN-gamma activity was neutralized with monoclonal antibodies (MoAb) to recombinant bovine IFN-gamma. IFN-gamma production was apparently suppressed by treatment of the PBMC with MoAb to bovine pan-T cell. IFN-gamma was more susceptible to treatment with MoAb to CD4+ than that with MoAb to bovine CD8+, suggesting that the CD4+ T-cell subset was the major producer of the lymphokine formed in the culture. IFN-gamma production by concanavalin A (Con A) stimulation was found to be significantly depressed throughout the perinatal period (from 5 weeks before to 7 weeks after parturition) when compared to that of the non-pregnant control group, but phytohemagglutinin (PHA)-induced IFN-gamma production was markedly depressed from 3 weeks before to 2 weeks after calving. The addition of recombinant human interleukin 2 (100 IU/ml) to the PBMC cultures from perinatal cows resulted in the augmentation of IFN-gamma production by stimulation with Con A or PHA.