Ia antigens isolated from radiolabeled spleen cells by immune precipitation from detergent extracts using subregion specific anti-Ia sera showed superficial structural similarity. Under nonreducing conditions, subregion-specific differences were found. The α(∼35,000 dalton) and β(∼25,000 dalton) chains of Ia molecules determined by the I-A subregion of haplotypes b, d, f, k, g, r and s were found partly in a disulfide linked form (∼58,000 daltons, “58K”) but those determined by the I-C subregion of haplotypes d, k, p and r were not linked. Reduction of isolated 58K material produced electrophoretic peaks equivalent to α and β.

The I-A determined molecules were distributed between 58K and α + β forms, and the ratio of 58K to α + β could be reduced by alkylating before detergent solubilization. It is not yet clear whether the I-A determined molecules which assume the 58K form are different from those which remain in the α + β forms.

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