The plasma levels of immunoglobulins IgA, IgG and IgM have been measured in 35 British, 44 Hawaiian-Japanese and 37 Japanese healthy adult women. Previous investigations showed that the mean levels of all three immunoglobulins were higher in Japanese than in British normal women. The present study finds that Hawaiian-Japanese women have "Japanese" levels of IgA, "British" levels of IgM and are intermediate for IgG. Thus, plasma IgM concentrations correlate with breast cancer incidence rates in the three racial groups and the reduced amounts of plasma IgM found in Japanese patients with breast cancer support this association.