An ascites subline (AA) of the murine sarcoma MC1M grows in vivo in the peritoneal cavity but dies in vitro when cultured on glass or collagen. The viability of AA cells in vitro is not influenced in cocultures with fibroblast cell line L929, and is diminished in cocultures supplemented with macrophage culture supernatant or in cocultures with non-adherent peritoneal cells. However, AA cells proliferate in vitro on glass or collagen when cocultured with syngeneic, semisyngeneic, and allogeneic peritoneal macrophages. This was demonstrated by tritiated thymidine incorporation assay, by AA cell number counting, and by measuring AA cell protein content. Proliferation also occurs when AA cells are separated from the macrophage monolayer by millipore filters.