Sequential immunoprecipitation analyses of Ia antigens were performed with 125I-labeled B10, B10.A(3R), and B10.A(5R) spleen cell lysates. The results suggest the presence of one I-region associated (Ia) locus (Ia-1) in the I-A subregion. In addition, they indicate that two Ia loci map between the I-J and S regions. One locus, Ia-5, maps in the I-E subregion. The second locus, most likely Ia-3, maps in either the I-E or I-C subregion. Taken together, the data demonstrate that at least three Ia loci in the I-region of the H-2 complex code for molecules detectable by immunoprecipitation.