The possible involvement of soluble cytotoxic factors (SCF) in the lytic mechanism of natural killer (NK) cell-mediated cytotoxicity (CMC) was investigated. Tumor target cells (TC) were examined for their ability to stimulate the release of SCF from human peripheral blood lymphocytes (PBL). SCF were detected when effector cells (EC) were stimulated with mycoplasma-contaminated (MC)-TC and with concanavalin A (Con A), but mycoplasma-free (MF)-TC were unable to induce SCF. Pretreatment of EC with beta-interferon augmented the production of SCF with MC-K562 and Con A, but not with MF-K562. However, MF-K562 once infected with the culture supernatant from MC-K562, as well as mycoplasmas concentrated from culture supernatant, were able to induce SCF. Only Con A was capable of generating cytotoxic supernatants from EC that had been inactivated with MF-K562 and from EC depleted of Leu 11b positive cells. Moreover, when EC were evaluated for NK activity after coculture with MF-K562, their ability to kill fresh MF-K562 in the standard NK-CMC assay was greatly reduced. This loss of lytic potential was not accompanied by SCF release. Collectively, these data suggest that mycoplasma organisms play a crucial role in the induction of human SCF, and the implication that SCF, including NK cytotoxic factors (NKCF), are the lytic mediators in NK-CMC needs to be re-evaluated.