Mouse and human lymphoid cells were cultivated in the presence of T-cell growth factor (TCGF) and evaluated for their in vivo anti-tumor effect in mice. Cultured spleen cells of normal BALB/c mice or of mice bearing the M109 tumor had a high level of cytotoxic activity in vitro against a variety of tumor target cells and had characteristics of natural killer cells. These cultured cells were evaluated for their in vivo cytotoxic activity by a mixture with [125I]dUrd-labelled M109 tumor cells (2 x 10(5)) at a 30:1 ratio and inoculation of the mixture into the footpads of BALB/c mice. The level of radioactivity remaining in the footpad was determined at various periods following inoculation of radiolabelled tumor cells. The presence of cultured cells in the inocula caused a marked decrease in the footpad radioactivity by 24 h. However, the slope of clearance of the radiolabel then became similar to that in the control mice. The cultured cells delayed, but did not prevent, tumor appearance, and did not influence the subsequent rate of growth of the tumors. The transient effects of the cultured cells contrasted with the more prolonged in vivo effects of alloimmune lymphocytes, and this may be due to their short survival period. Ninety-nine percent of [125I]dUrd-labelled cultured mouse lymphoid cells were eliminated within 48 h of i.f.p. inoculation. Cultured human lymphoid cells, initiated from the blood of normal donors, also had a high level of cytotoxic activity in vitro and were evaluated for their in vivo effects by intra-footpad inoculation into nude mice, together with radiolabelled human tumor cell lines, G-11 or HT-29. In vivo cytotoxic activity of the human cultured lymphoid cells correlated with their cytotoxic effect in vitro. These results indicate that cultured mouse an human effector cells have appreciable in vivo cytotoxic effects against tumor cell lines. However, the transient duration of these effects may limit their immunotherapeutic potential.