We have biochemically characterized by 2D (two-dimensional) electrophoresis three novel class I molecules called A166, A149 and A216 expressed by 1591, a UV-induced fibrosarcoma, and have compared them to class I molecules expressed by mice of the H-2q and H-2s haplotypes. A166 and A149 are very similar if not identical to Dq and Lq respectively. We have shown, using HPLC (high-pressure liquid chromatography) tryptic peptide mapping, that the expression of A166 is approximately three fold greater than A149, reminiscent of Dd compared to Ld. In addition A216 possess an identical isoelectric point to that of the Ks molecule. We demonstrate that outbred Swiss Webster mice express an analogous constellation of class I molecules and we conclude that our results can be most easily interpreted in terms of an allogeneic origin for the novel class I molecules expressed on 1591.