A rat X mouse T cell hybrid (PC60) proliferates in the absence of T cell growth factor (TCGF) and its cytolytic activity can be induced by culture in mixed leukocyte culture supernatants or concanavalin A-activated rat spleen cell supernatant (CS) to lyse 51Cr-labeled tumor target cells. To characterize the factor(s) responsible for this reversible induction, serum-free CS was fractionated by reverse phase high performance liquid chromatography and by phenyl-Sepharose chromatography. A cytotoxicity-inducing activity (CIA) was separated from TCGF and macrophage-activating factor/interferon-gamma. CIA was found to be a macromolecule with an apparent molecular weight of 12,000-18,000 and a pI of 5.0 and 6.2. Its activity on PC60 cells depended on the addition of TCGF. Thus TCGF may have other effects on T cells than the induction of entry into cell cycle. The number of TCGF surface receptors on PC60 cells was measured using purified 3H-TCGF. TCGF receptors were undetectable on noninduced cells but appeared during induction. The expression of TCGF receptors was not induced either by TCGF or by CIA-containing supernatants or fractions alone, only by a combination of both. These results show that TCGF plays a role in the regulation of the expression of its own receptors.