A study was conducted in dairy cows to evaluate the efficacy of recombinant bovine interleukin 2 (rBoIL-2) as an adjunct to antibiotic therapy in Staphylococcus aureus mastitis. In normal, non-mastitic cows, intramammary infusion of rBoIL-2 caused a tenfold increase in somatic cell counts (SCC) in milk. Co-administration of 2 mg of rBoIL-2 and sodium cephapirin in cows with established S. aureus mastitis decreased SCC and shedding of S. aureus compared with values from cows that were given only sodium cephapirin or 10 mg rBoIL-2 with sodium cephapirin. Cows in the 2 mg rBoIL-2 group cleared the infection earlier and at 2 weeks after treatment had not relapsed with staphylococcal mastitis. These data suggest that rBoIL-2 may be useful as an immunotherapeutic agent in controlling mastitis.