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In an attempt to identify the role of selective pressures in the maintenance of immunoglobulin polymorphisms, the phenotypes identified by the immunoglobulin allotypes Glm(l), Glm(2), Glm(3), G3m(5) and Km(l) have been determined in a large sample of adult Caucasians resident in Melbourne, Australia. While no evidence for age or sex-related differences was found, non-random distribution of Gm and Km phenotypes suggests the existence of selective forces favoring Gm heterozygotes who are Km (1 + ) and Gm homozygotes who are Km(l —).