Certain non-H-2 alloantigens are associated with murine B-lymphocyte Fc receptors in that pretreatment of spleen cells with alloantibodies against these antigens inhibited binding of Ig complexes to B-cell Fc receptors. This inhibition was specific in that: (a) as has been shown previously, the Fc portion of the alloantibody was not required to produce inhibition; and (b) antibodies against some non-H-2 antigens but not antibodies against others (including some that were expressed on B cells) caused such inhibition. Backcross experiments revealed that the B-cell Fc receptor-associated non-H-2 antigens were determined by the gene(s) of a single background locus in each of the three strains tested (A/J, B10, and CBA/J). This locus was poly-morphic in that at least four different B-cell Fc receptor:associated non-H-2 antigens were identified (one each in the A/J, B10, and CBAJJ and one antigen shared or crossreactive between the B10 and CBA/J). These antigens were primarily but not exclusively expressed on B lymphocytes as determined by immunofluorescence studies, and on the basis of capping experiments they did not appear to be identical to B-cell Fc receptors. Linkage studies revealed that the locus which determined these antigens was not linked to the albino locus nor the heavy chain allotype locus and expression was neither sex-limited nor an X-linked recessive trait. However, this locus was closely linked but not identical to the Mls locus (apparent recombination frequency 6.8 percent). Thus, two closely linked non-H-2 loci both determine the expression of antigens which have characteristics similar to Ia antigens. One locus is polymorphic and determines the expression of antigens which are primarily expressed on B cells and are specifically associated with the Fc receptors of these cells. The other (Mls locus) determines antigens which are stimulatory in mixed lymphocyte cultures. These observations suggest that there may be a second gene complex which is the analogue of the I region of the H-2 complex.