Three kinds of mouse tumor necrosis factor (TNF), which have molecular weights of 35 kDa, 45 kDa, and 18 kDa on SDS-PAGE, were partially purified from serum-free culture supernatants of mouse peritoneal macrophages induced with lipopolysaccharide. Analysis of the native molecular weights by gel filtration indicated that the 18 kDa and 45 kDa TNFs aggregate into 50 kDa and 100 kDa molecules, respectively, while the 35 kDa TNF is contained in high molecular weight aggregates of approximately 200 kDa. The three kinds of cytotoxic factors all elicited tumor reducing responses.