Evidence for a sex-linked determinant of immunoglobulin M levels was sought using correlational and commingling analyses in a sample of 174 randomly selected nuclear families. While mean IgM levels in females were approximately 25% higher than that in males, the pattern of familial correlations did not follow the expectations under a sex-linked model, and there was no commingling in the distribution of IgM levels as expected when a trait is under the influence of a major gene.