The Mls locus was originally defined to have four alleles; three controlled products that were detectable in primary mixed leukocyte reactions (MLR), whereas one, b, was described as being null. Recently, other investigators postulated that the Mls locus is nonpolymorphic, being composed of the b null allele and of a singly expressed allele previously thought to be the a and d alleles. We previously reported that products controlled by Mls a and Mls d were antigenically distinct and therefore are not controlled by the same allele, and the product of Mls b on cells of three different strains was easily detectable by Mls ~ and Mls ~ responding cells. Thus the b allele is not null. In the present report evidence is presented which indicates that both Mls b and Mls ~ encoded products were undetectable by MLR when in the presence of Mls ~ or Mls d. This was demonstrated by (a) the inability of Mls"/Mls c and Mlsa/Mls b F 1 cells to stimulate Mls" responding cells and Mlsd/Mls ~ and Mlse/Mls b cells to stimulate Mls ~ cells; (b) the positive response of Mlsa/Mls b and Mlse/Mls b Fl-hybrid cells to Mlsb-encoded products; and (c) the reactivity of Mls"/Mls ~ and Mlsd/Mls c F 1 hybrid cells to Mls~-encoded determinants.