The L1210 murine lymphoblast cell line possessed a B lymphocyte-specific alloantigen which was detected with C57BL/Ks anti-L1210 serum. The antigen was found on splenic B lymphocytes but not on thymocytes, T lymphocytes, or erythrocytes. It was present on only 7% of bone marrow cells. The reactivity of C57BL/Ks anti-DBA/2-spleen serum was indistinguishable from that of the anti L1210 serum, confirming that the antigen was a normal component of B lymphocytes of the CBA/2 mouse. The strain distribution pattern of the antigen detected by the C57BL/Ks anti-L1210 serum indicated that this alloantigen was not an allele of the H-2K, H-2D, Mls, or Ig loci. Genetic analysis indicated that the antigen was inherited as a single, Mendelian dominant trait. Segregation analysis of (B10.BR X DBA/2)F1 X AKR/J offspring indicated that this B cell marker was not linked to geneslambdacoding for H-2.31 or Ly 4.2. The C57BL/Ks anti-L1210 serum identified a new polymorphic genetic locus, the product of which was B lymphocyte specific.