Suspensions of PMN isolated from milk and S. aureus were incubated together at different ratios and with different types and concentrations of mammary fluids. Results showed that greater numbers of S. aureus were left unphagocytosed and alive within PMN at the higher S. au-reus: PMN ratios (2:1) than at the lower ratios. This effect was more noticeable in whole milk than in skimmed milk. Phagocytosis and destruction of S. aureus by PMN (ratio 1:10) were reduced (p < 0.01) to a greater extent in whole milk or in skimmed milk containing 10% cream than in either skimmed milk or whey. There was no difference between skimmed milk or whey. Increasing the concentration of either whole milk or skimmed milk in the incubation mixture from 10 to 40% resulted in fewer (p < 0.01) S. aureus phagocytosed but had no affect on intracellular kill. The data show that the milk fat globule is a major deterrent to the phagocytosis and destruction of S. aureus by PMN. Casein inhibited phagocytosis, but only when the concentration of skimmed milk in the incubation medium was increased to 40%. Casein did not affect intracellular kill.