The anti-tumor effect of syngeneic spleen cells, xenogeneic immune RNA (l-RNA) and tumor antigen (TA) was studied in a chemically induced (N-methyl-N-nitro-sourethane) colon carcinoma model in BALB/c mice. When the mice were treated, sequentially, with these three elements by local injection under the tumor, complete tumor regression was observed in 25% of the animals. Complete tumor regression was observed in 17% of animals treated by injection of spleen cells pre-incubated with l-RNA in vitro followed by an injection of TA. When spleen cells were treated first with l-RNA and then with mitomycin-C, therapeutic benefit was obtained and the survival rate of mice treated with these spleen cells and TA was significantly higher than that of the control group (no treatment). The anti-tumor effect of this treatment was abrogated completely when spleen cells were treated with mitomycin-C prior to l-RNA incubation. When spleen cells were incubated with TA in vitro or after incubation with l-RNA and then injected into tumor-bearing mice, no anti-tumor effect was noted. These data suggest that DNA synthesis and/or lymphocyte proliferation is essential for the action of l-rna in sensitizing lymphocytes but not essential in the interaction of l-RNA-sensitized lymphocytes with TA. In addition it appeared that, under the conditions used, the participation of host factors was essential in the latter interaction, since treatment of l-RNA-sensitized lymphocytes with TA in vitro was ineffective.