Further analysis of the isoantigens (allotypes) of 2 classes of normal mouse immunoglobulins, gammaG(2a) and gammaG(2b), has shown a minimum of 10 specificities for the Ig-1 locus (controlling gammaG(2a)-antigens) and 3 specificities for the Ig-3 locus (controlling gammaG(2b)-antigens). Three gammaG(2)-myeloma proteins of plasma cell tumors induced in (NZB x BALB/c)F(1) mice have been analyzed for the isoantigens they carry. NZB mice are genotypically Ig-1(e) Ig-3(e), while BALB/c are Ig-1(a) Ig-3(a). Two of the myeloma proteins are gammaG(2a)-globulins. One of these, GPC-7, carries all the isoantigenic specificities of the Ig-1(e) allele while the other, GPC-8, carries all the isoantigenic specificities of the Ig-1(a) allele. Thus only one of the parental alleles of the mouse in which the tumor arose is expressed in each of these myeloma proteins. The third myeloma protein GPC-5, also carries the antigens of only one parental strain (NZB). However GPC-5, a gammaG(2b)-globulin, carries only one of the Ig-3 specificities normally associated with gammaG(2b)-globulins of NZB. Most remarkably it also carries one Ig-1 specificity normally associated with gammaG(2a)-globulins of NZB. This is the first analyzed mouse myeloma shown (a) to express some but not all the antigenic specificities normally associated with an allele and (b) to carry antigenic specificities controlled by two distinct immunoglobulin loci. The implications of these findings are discussed in relation to the genetic control of immunoglobulins.