SummarySerum gamma globulin(Gm)groups were determined for 262 Japanese. The resulting allele frequency estimate forGmawas 0–885 and forGmb0–115. Some reservation is necessary in accepting these estimates at face value because of failure to observe any Gm(a ‐ b +) individual.There was no significant difference in theGmaandGmbfrequencies between normal Japanese subjects and Japanese with rheumatoid arthritis.An attempt was made to detect new Gm factors in the American population by the use of sera from Japanese rheumatoid arthritis. The attempt was based on the assumptions that effective typing sera from rheumatoid arthritics must lack the Gm factor sought, that such typing sera must be very rare in populations where the Gm factor in question is common, and that ethnic differences inGmallele frequencies extend to undetected alleles. The attempt failed; however, the method of approach may be of value to future investigators.We wish to offer our thanks for the provision of sera from American‐born Japanese children to Dr Stanley Wright, University of California at Los Angeles, School of Medicine.This investigation was supported in part by Grant no. B2053 from the National Heart Institute, the National Institutes of Health, Bethesda, M