Six nonpregnant, nonlactating Jersey cows, averaging 4 to 6 years old, were used to evaluate the immunomodulatory effects of recombinant bovine interleukin 1 beta (rBoIL-1 beta). Cows were given 166 ng of rBoIL-1 beta/kg of body weight at 8-hour intervals for 96 hours. Persistent leukocytosis was observed within 3 hours of rBoIL-1 treatment, peaking 24 hours after the first IL-1 beta injection and returning to baseline values within 72 hours after cessation of IL-1 beta treatment. Injection of cows with rBoIL-1 beta stimulated lymphocyte blastogenesis and mitochondrial methyl-thiazoltetrazolium cleavage activity in resting cell cultures. Increases in the aforementioned lymphocyte activities were also observed in stimulated blood mononuclear cell cultures during IL-1 beta administration. Change in IgM production in cell cultures was not observed during IL-1 beta treatment. Within 24 hours of the first IL-1 beta injection, IL-1 beta mRNA transcription in stimulated blood mononuclear cell cultures was markedly increased, suggesting that IL-1 beta upregulates its own production in mononuclear cells. These data provide evidence that administration of cytokines, such as rBoIL-1 beta, enhances immune cell function and, therefore, may be useful in alleviating immunosuppresion in cattle.