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Blood groups and serum protein genetic systems have been investigated in Blackfeet Indian samples, drawn from the Blackfeet Reservation, Montana, U.S.A. Comparisons of demographic characteristics of the sample with the population from which it was drawn suggest bases for systematic bias. The influence of non-random ethnic exogamy and out­migration on genetic compositions of reservation populations is discussed, relative to the Blackfeet and Blood divisions of the Blackfeet tribe. Comparison of phenotypic distributions within the two divisions suggests possible genetic heterogeneity. The presence of the transferrin variant B0-i within the Blackfeet population is documented and related to prob­able gene flow from southwestern U.S. or Mexican sources.