Guinea pig lymphocyte antigen molecules Ia.3,5 and Ia.8 share structural homology expected for alleles. 
Because of the lack of I region recombinant guinea pig strains, rigorous comparative chemical analyses of guinea pig Ia alloantigens have been utilized to gather structural information that would allow tentative assignment of I region alleles.  In this study, the B/Lac Ia.8 molecule was chemically compared to the strain 13 Ia.3,5 molecule and the strain 2 Ia.2 and Ia.4,5 molecules.  SDS-PAGE, IEF, and Cleveland peptide mapping demonstrated a significant degree of homology between the Ia.8 and Ia.3,5 alpha-chains.  HPLC mapping of trypsin/chymotrypsin cleavage products revealed an overall coincidence of peptides of 57 and 68% for the comparisons of the Ia.8 and Ia.3,5 alpha- and beta-chains respectively.  In contrast, comparisons of Ia.8 and Ia.2 alpha- and beta-chains exhibited a significantly lower degree of similarity--39 and 47% respectively.  The degree of homology seen in the Ia.8 and Ia.3,5 comparisons is consistent with those values seen in comparisons between the Ia.3,5 and Ia.4,5 molecules (putative allelic products of the guinea pig I region) and with those values obtained for allelic products of the mouse H-2 system.  The results of this investigation strongly suggest that the Ia.8, Ia.3,5, and Ia.4,5 molecules are the products of alleles at the Ia alpha- and beta-chain loci.