SR/CR macrophage killing of S180 cells partially depends upon the release of serine proteases. Purified SR/CR macrophages were pre-incubated with trypsin inhibitor from bovine pancreas at concentrations of 250, 1000, or 4000 U/ml to inhibit serine proteases. The macrophages were then cultured with S180 cells in standard conditions to analyze the in vitro killing ability of serine protease-inhibited macrophages compared to uninhibited controls. Results are reported as the mean and standard deviation of S180 cell killing and represent 2 separate experiments. The statistical significance of inhibition of killing as compared to that in uninhibited control samples was determined by the Student’s t-test. Statistically significant values were defined as P less than or equal to 0.05, and are denoted by a *.