Peritoneal exudate macrophages from normal, untreated or thioglycollate-elicited mice, lysed syngeneic fibroblasts and lymphoblasts modified by 2,4,6-trinitrobenzene sulphonic acid (TNBS) in vitro. Optimal lysis of the hapten-modified cells by elicited macrophages was usually seen after 18 hr of co-cultivation at E:T ratios of 10:1-30:1. Cytotoxicity was expressed by macrophages depleted of T cells, and was not potentiated by LPS. Allogeneic TNBS-modified cells were lysed by non-immune, non-activated macrophages to the same extent as syngeneic modified targets, indicating that genetic restriction does not appear to play a role in macrophage-mediated cytolysis of TNBS-modified cells.