Lymphoid cells obtained from the spleen, thymus, bone marrow, peripheral blood, and peritoneal exudate of normal mice (BALB/c, BALB/c nude, C57BL/6, C3H) and from spleens of mice bearing a transplantable lung carcinoma or primary mammary carcinoma were expanded in culture for 1-9 months, with an increase in cell number of 10(5)- to 10(6)-fold per month, in crude or lectin-depleted medium containing T cell growth factor (TCGF). All these cultured lymphoid cell (CLC) lines exhibited strong cytotoxic activity in vitro (assessed by 51Cr-release assays) toward a variety of freshly harvested and cultured syngeneic, allogeneic, and xenogeneic tumor target cells, both lymphoid and solid (including metastatic growths) in origin. Extensive killing was observed against tumor targets that were resistant to lysis by natural killer (NK) cells as well as to NK-sensitive tumor lines. Low levels of cytotoxic reactivity were also demonstrated against fresh and cultured normal lymphoid cells. The CLC had some characteristics of NK cells but also expressed some typical T cell markers. In local Winn-type neutralization assays, CLC delayed or completely inhibited the growth of lymphomas and carcinomas in syngeneic and allogeneic recipients. In mice with metastatic growth of a second-generation transplant of mammary carcinoma, CLC were shown to have some therapeutic effect when administered IV 1 day after cyclophosphamide. No significant beneficial action of IV administered CLC was observed in the absence of chemotherapy in mice implanted with a lung carcinoma. The possibilities of employing TCGF-propagated cytotoxic effector cells in adoptive immunotherapy of human malignancies are discussed.