Gene products of the murine Mls locus elicit high primary mixed lymphocyte reactions (MLR). Although this locus is considered to have 4 alleles, only 2 of these alleles, Mlsla and Mlsd, give strong responses in primary MLR. A series of experiments are presented which show that Mlsa and Mlsd are in fact antigenically indistinguishable. Strong Mls determinants appear to be nonpolymorphic.