Isoantibodies can distinguish γ-globulins of different allotypic (antigenic) specificities existing among individuals of the same species (1–3). In mice, two such γ-globulin allotypic specificities have been identified (4–7). In a previous study, two mouse γ-globulin allotypic specificities, Asa1 and Asa2, were identified by ascitic fluid isoprecipitins using agar gel methods and were found to be inherited by codominant autosomal alleles (7). The sera of each of 38 inbred strains reacted with either the anti-Asa1 or anti-Asa2 antibodies but never with both (7). This paper presents the results of further immunochemical and genetic studies with new immune ascitic fluids and sera obtained by immunization of recipient inbred mice with γ-globulin of donor inbred mice of other strains. With the availability of antibodies of greater specificity from new donor-recipient combinations, the previously reported precipitin reactions of the anti-Asa1 with the sera of some of the inbred strains (7) were found to be the result of cross-reactions with three new γ-globulin allotypes Asa3, Asa4 and Asa5.