RNA fractions extracted from the spleens of immunized animals prepared against L1210 leukemia cells can transfer allospecific cell-mediated immunity both in vitro and in vivo. Both preculture of nonsensitized lymphocytes prior to the treatment with immune RNA and additional culture of lymphocytes after 1-hr treatment with immune RNA enhance the growth-inhibitory activity of immune RNA-treated lymphocytes. Preculture for 5 hr and additional culture for 24 hr are sufficient for maximal enhancement of the growth-inhibitory activity. There is no significant difference in inhibition between lymphocytes treated with immune RNA for 1 hr and 24 hr. The growth inhibition by lymphocytes is augmented proportionally to the increase in the dose of immune RNA in vitro but not in vivo. In mice given i.p. injections of immune RNA-treated syngeneic spleen cells (before or after inoculation with L1210 cells), a significant prolongation of the mean survival time [27.0 +/- 5.4 (S.D.) days, 0.05 < p < 0.1] was not achieved when compared with control mice (21.4 +/- 1.6 days). Evidence is presented that suppressor cells in the spleens of L1210-bearing mice are involved in preventing effector cell function in vivo. The present study revealed that 8 X 10(3) and 5 X 10(5) suppressor cells were sufficient to inhibit completely the activity of immune RNA-treated effector cells in vitro and in vivo, respectively. Suppressive activity was abolished by treatment with anti-Thy 1.2 serum and complement and partially abolished with irradiation (1500 R). The suppressor cells belonged to a radiation-sensitive T-cell population. The limited activity of immune RNA in vivo is probably due to the destruction of immune RNA by RNase present in the host plasma and tissues and the acquisition of suppressor cells which inhibits nonadherent peritoneal exudate cell activity in the animal.