The physiological effects of intramammary infusions of recombinant bovine cytokines in six lactating dairy cows on the quality and yield of milk and the bactericidal activity of milk neutrophils were investigated. Recombinant bovine interleukin-2 (rboIL-2) and interferon-γ (rboIFN-γ) were produced in the yeast Pichia pastoris. Two animals were given rboIL-2 (2×105 units) in two quarters, two animals were given rboIFN-γ (6·5×105 units) in two quarters, and the other two cows received a dose of rboIL-2 in one quarter and rboIFN-γ in a second quarter. In addition, each animal was given phosphate-buffered saline (PBS) in the other two quarters as a control. Somatic cell counts and conductivity of the fore milk were monitored before and after infusion. Neutrophils were isolated from quarter milk samples 36 h after infusion of cytokine or PBS and their bactericidal activities against Staphylococcus aureus were measured in vitro with a colorimetric assay. Quarters infused with rboIL-2 or rboIFN-γ showed significant but transitory increases in both milk somatic cell counts and conductivity when compared with preinfusion values and with control quarters. There were minimal effects on daily milk yield. Neutrophils isolated from milk from quarters infused with rboIL-2 showed enhanced bactericidal activity against Staph. aureus. The bacterial killing from rboIL-2 treated quarters was significantly greater, with a mean of 63·5% compared with a mean of 5·4% for neutrophils taken from uninfected quarters to which PBS had been administered. The bactericidal activities for quarters treated with rboIFN-γ and infected quarters treated with PBS were 15·0 and 30·0% respectively. The results indicate that intramammary infusions of rboIL-2 and rboIFN-γ to lactating cows are well tolerated, and that rboIL-2 can activate milk neutrophils and augment their bactericidal activity.