Pretreatment of human peripheral blood lymphocytes with interferon significantly enhances the release of lymphotoxins (LTs) observed at subsequent incubation of the cells, for 3 hr, with phytohemagglutinin (PHA). Fractionation of the LTs by gel filtration shows that interferon (IFN) strongly increases the release of certain LTs produced by these cells, while it has little effect on the release of others. The release of LTs from the IFN-treated cells is dependent on stimulation by PHA, requires Ca2+ ions, and can be blocked by prostaglandin E1, but it is independent of protein synthesis.