Macrophage cell lines produce a cytotoxin

Cytotoxic activity, measured by release of tritium from 3H-(methyl)-thymidine-labeled 7943 cells, was found in culture fluids from four of five macrophage cell lines. In the P388D1 cell line, cytotoxic activity was highest after 7 days of culture, 2 days after the cells reached confluence. The activity was not lost upon dialysis or lyophilization. When culture fluids were chromatographed on Sephadex G-200, two activity peaks were obtained: one in the void volume and the other corresponding to a m.w. of 40,000 to 50,000.