Allogenetically sensitized T lymphocytes secrete in the presence of the specific antigen a factor which renders macrophages cytotoxic to target cells in vitro, macrophage cytotoxicity factor. Cytotoxicity is expressed as 51Cr release from labeled target cells. Experiments are described to characterize chemically nonspecific MCF activity. After purification on a DEAE-cellulose column, active material elutes from Sephadex G-100 in a single peak. Assaying individual fractions of this peak shows that nonspecific MCF activity is found at a mol. wt. of 28,000+/-5,000. This material is stable at -70 degrees C several months.