Class II molecules of 17 B10.W lines (i.e., congenic lines carryingH‐2haplotypes of wild mice on C57BL/10Sn or B10 strain background) were analyzed by immunochemical methods of precipitation and co‐precipitation. The analysis included typing for previously defined antigenic determinants, as well as determinants defined by new reagents, and mapping of the genes controlling these determinants. Using this approach, an Ia chart of the typed B10.W lines could be constructed. Genes controlling determinants Ia.38, 102, 103, 104, 105, and 106 were mapped intoA(AβorAα) locus; genes controlling determinants Ia.41, 107, 108, and 109 were mapped in theE(EβorEα) locus. The study suggests that the polymorphism of at least some of the class II loci could be as high as that of the class