A weakly positive but statistically significant association between HLA-Cw5 and myeloma has been reported in black patients. The authors attempted to determine whether immunoglobulin allotypes of the Gm series demonstrate any such association. They were identified in the sera of 29 black patients and 160 healthy black control subjects by a standard hemagglutination-inhibition technique. The results indicate that the G3m(g5) allotype is significantly associated with myeloma. Furthermore, addition of that immunoglobulin allotype to a Gm phenotype that is negatively associated with myeloma gives a phenotype that is positively associated with the disease, both associations being statistically significant. It was concluded that G3m(g5) is a marker of inherited susceptibility to myeloma in black Americans. Furthermore, as G3m(g5) is present in almost 50% of normal control subjects, it was proposed that its expression in a much greater proportion of patients may be related to an underlying genetic rearrangement that is also associated with neoplastic transformation.