Alloreactive T lymphocytes are readily detected in unprimed animals although they have never encountered the alloantigen before. This well-established phenomenon is usually explained with the assumption that a self-MHC molecule complexed with a defined peptide resembles the allo-MHC molecule with another peptide and induces the corresponding T cell specificities. Here, for the first time and in support of this hypothesis, self-MHC-restricted peptides are described for a T cell clone that was induced with allo-MHC. The allo-MHC-specific CTL clone 2C was derived from a H-2b mouse and recognizes H-2Ld complexed with the naturally occurring endogenous peptide LSPFPFDL.