Normal resident peritoneal macrophages from C3D2 (C3H/Tif X DBA/2) F1 mice were activated in vitro by culturing with semisyngeneic tumour cells. The tumour cells originated from a methylcholanthrene-induced sarcoma (MC1M) growing in vivo in ascites form. Macrophage-mediated cytotoxicity was evaluated after 5 days of in vitro culture, using five different target cells. Semisyngeneic (L 929), allogeneic (B16 melanoma), and xenogeneic (HeLa) tumour cell lines and normal allogeneic fibroblast cell lines (3T3, 3T6) were tested. The morphology and kinetics of the cytotoxicity reaction were studied by scanning electron microscopy and compared with release of radioactivity from 14C-thymidine-labelled target cells. The activated macrophages were able to kill the semisyngeneic, allogeneic, and xenogeneic tumour cell lines tested under conditions that did not affect normal fibroblasts. The requirement for T cells during activation of the macrophages was also tested. The cytotoxicity decreased markedly when T cells were removed from the macrophage cultures before activation or when macrophages from nude mice were used in the experiments.