SummaryIn an attempt to characterize the effector cells responsible for tumour resistance inSalmonella enteritidis11RX‐immunized mice the anti‐macrophage agent trypan blue was used in bothin vivoandin vitroexperiments. Resistance was measuredin vivoby the clearance of125I from she peritoneal cavity of mice injected intraperitoneally with125I‐5‐iododeoxyuridine labelled Ehrlich Ascites Tumour (EAT) cells. Thein vitrocorrelate was measured by lysis of51Cr‐labelled tumour cells by peritoneal cells (PC) from 11RX‐immunized mice. Pre‐treatment of resistant mice with trypan blue greatly reduced both125I clearance and51Cr release. Thein vitrocytolytic activity was non‐specific.Fractionation of cytotoxic PC on the basis of adherence to plastic or nylon wool and buoyant density, coupled with the use of appropriate cell targets, showed that the bulk of cytotoxic activity resided with macrophages, with some contribution from other cells such as natural killer cells.Killing of labelled tumour cells could be inhibited by competition with unlabelled cells or by separating the PC and tumour cells by a cell impermeable membrane. This showed that close association between the effector and target cells was necessary before killing could occur.