Negro and Caucasian samples typed with anti-M, N, S, s, M(1), Ua, Hu, He, Sj, and Tm indicate at least 14 and perhaps more than 26 alleles at the MNS locus, for which a notation is proposed. Gene frequencies arc estimated by maximum likelihood, using the Alltype computer program. The methodology, uses, and limitations of such estimates are discussed. There is a clear distinction between idiomorphs (with frequencies less than 0.01), many alternative sets of which can account for the rare phenotypes with or without typing errors, and polymorphs (alleles with frequencies between 0.01 and 0.99) which cannot fail to be recognized in a large sample. Idiomorphs require confirmatory family studies.

Summary. Negro and Caucasian samples typed with anti‐M, N, S, s, M1, Us, Hu, He, Sj, and Tm indicate at least 14 and perhaps more than 26 alleles at the MNS locus, for which a notation is proposed. Gene frequencies are estimated by maximum likelihood, using the ALLTYPE computer program. The methodology, uses, and limitations of such estimates are discussed. There is a clear distinction between idiomorphs (with frequencies less than 0.01), many alternative sets of which can account for the rare phenotypes with or without typing errors, and polymorphs (alleles with frequencies between 0.01 and 0.99) which cannot fail to be recognized in a large sample. Idiomorphs require confirmatory family studies.

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