Fifteen (CBAxCBA/H-T6) in equilibrium C57BL/6 allophenic mice were analyzed for the parental composition of their spleen white blood cells and serum immunoglobulin allotypes. The spleen compositions, assessed by a microcytotoxicity test, showed that the animals covered a range of parental cell mixtures. The allotype compositions of the IgG2a and IgG2b subclasses were determined by a solid phase radioimmunoassay. Although the absolute amount of IgG2a and IgG2b varied markedly from animal to animal, the percentage of the two allotypes in each subclass was remarkably constant. This suggests that the synthesis of IgG2a and IgG2b may be under a coordinate control system. Also, the spleen cell compositons were in excellent agreement with both the IgG2a and IgG2b subclass compositions. The quantitative analysis of the immune systems of allophenic mice may be first step toward the understanding of the mechanisms of tolerance of histoincompatible cells in allophenic mice.