To ascertain whether guinea pig Ia antigen-bearing molecules might act as receptors for exogenous antigens, Ia molecules from strain 2 and strain 13 guinea pigs were tested for the presence of variable regions by isoelectric focusing (IEF). Ia antigens from lymph node cells, T lymphocytes, and macrophages were radiolabeled with 3H-leucine or 35S-methionine, solubilized by NP-40, and isolated by immunoprecipitation. The IEF characteristics of each component Ia chain were studied by employing a “reverse” O'Farrell technique whereby the two Ia chains were first separated by SDS-polyacrylamide gel electrophoresis, and then each chain was subjected to IEF. Comparison of Ia molecules derived from each of the cell populations, but bearing identical Ia determinants, revealed virtually identical IEF patterns, suggesting that the same class of Ia molecules can be derived from each cell population studied. Within each inbred strain, Ia molecules bearing different determinants had similar but distinctive IEF patterns.