Direct contact is required for SR/CR macrophages to kill S180 cells. SR/CR (abbreviated SR) and WT macrophages were purified. The role of contact between macrophages and S180 cells was analyzed by incubating the cell types either directly together or in adjacent wells of Transwell plates, separated by a 0.4 micron filter. Cells were incubated in standard in vitro killing assay conditions as described previously. For each condition, S180 cells were analyzed for survival by trypan blue exclusion and intact morphology and compared to a control without macrophages. Results are reported as the mean and standard deviation of percentage of S180 cell survival for at least 3 experiments.