This study reports the presence of a factor in human milk which is cytotoxic to both autologous and heterologous human peripheral blood mononuclear cells (PBMC). The cytotoxic effect was measured by the release of 51Cr from labelled cells or by the inability of PBMC to exclude trypan blue following exposure to milk. The cytotoxic factor was not dialysable or sensitive to heating at 56 degrees C for 30 min, and could not be adsorbed from milk by PBMC. It lysed lymphocytes harvested from colostrum and from autologous or heterologous milk. The cytotoxic factor in the milk was different from the factor found in colostrum which inhibits the proliferation of mitogen stimulated PBMC, but which is not cytotoxic. Testing of serial samples of milk from the same mother indicated that the factor could be detected in the milk after 3-4 days post partum. We conclude that factors cytotoxic for human PBMC appear in human milk early in lactation.