Self-peptide pools eluted from purified H-2Kk or H-2Kkm1 molecules were sequenced. The majority of self-peptides associated with H-2Kk molecules were found to be octamers with two anchor positions. Position 2 is invariantly occupied with Glu, and the C-terminal residue at position 8 is almost always Ile. Comparison of this motif with synthetic peptides known to contain viral or parasite T-cell epitopes could be well aligned with this motif, except that the C-terminal Ile residue frequently appears to be at position 9 of the aligned sequences, instead of 8. Self-peptides eluted from mutant H-2Kkm1 molecules also appear to be mostly octamers with the same Ile residues at position 8 as with Kk. Position 2 is still dominantly occupied by Glu; in contrast to the Kk motif, however, Gln, Gly, and Pro are also allowed. Other differences between the two motifs indicate that a certain number of peptides presented by one of the molecules are not presented by the other.