C.B-20 ( Ighb ) mice challenged with BALB/c ( Igha ) spleen cells (or vice-versa) generate cytotoxic T lymphocytes (CTL) that recognize an antigen, H-40, controlled by an Igh-linked gene. The gene maps to the Igh-C region end of the Igh complex, telomeric to Tsu in the region of Pre-1. At least three alleles, a, b, and c, can be defined. Using a cold target competition assay, no polymorphism of the a allele was detected. Both surface Igh-5a positive and negative spleen cells from (C.B-20 X BALB/c)F1 animals express the a allele of the antigen, indicating that this gene is not allelically excluded. Recognition of the target antigen by CTL is restricted by the D-end of H-2d. The tissue distribution of H-40 was explored using both bulk-cultured and cloned CTL. The antigen is expressed on surface immunoglobulin positive (sIg+) cells and correlates with the expression of sIgM. This was determined by analysis of several B lymphomas as well as of other tumors that varied in their extent of expression of sIg. Four subclones of BCL1 were analyzed. Two of the subclones are sIg+ and express H-40, while two other subclones are sIg- and H-40-. Thus, these data define an Igh-linked gene, separate from immunoglobulin structural loci, that controls an antigen expressed on sIg+ cells. Possible mechanisms to account for this finding are discussed.