T cell recognition of transforming proteins encoded by mutated ras proto-oncogenes. Activated ras proto-oncogenes contribute to the pathogenesis of many animal and human malignancies. ras proto-oncogenes are generally activated by point mutations within codons 12 or 61, which result in the expression of ras protein (p21) bearing characteristic single amino acid substitutions at the corresponding residues. The purpose of the current study was to determine whether the presence of single transforming amino acid substitutions can render normal ras protein immunogenic and, thus, a possible target for T cell-mediated tumor therapy. In initial experiments, C57BL/6 mice were immunized with a synthetic peptide corresponding to residues 5 through 16 of p21 containing the transforming substitution of arginine for normal glycine at residue 12. The results demonstrated that class II MHC-restricted T cells which were specific for the peptide could be elicited, and that the peptide-induced T cells could specifically recognize the corresponding intact p21 ras protein. Recognition of p21 ras protein by peptide-specific T cells implies that C57BL/6 APC can process the activated ras protein in a fashion that allows presentation of digested protein by class II MHC molecules in a configuration similar to the configuration with synthetic peptide. Evaluation of the immunogenicity of peptides containing alternative transforming amino acid substitutions of ras protein demonstrated that some, but not all, were immunogenic in individual strains of mice. Therefore, although ras protein-specific T cells can be elicited by immunization with synthetic peptides, not all of the potential ras mutations commonly associated with malignancy may be recognizable by T cells from all individuals. 