Data are presented on electrophoretic variants of 25 polypeptides found in the blood serum and erythrocytes, in 812 individuals from three Amerindian tribes, the Pano, the Baniwa, and the Kanamari. Two “private polymorphisms” were encountered, of PEPB in the Pano and CAII in the Baniwa. A single example of a different PEPB variant was encountered in the Baniwa, and two possible examples of an unstable variant of HGB A2 in the Kanamari. In addition, the well-known A variant of ACP1, the Duarte variant of GALT, the 2 variant of Hp and the 2 variant of PGM1 occurred in polymorphic proportions in all three tribes, and the TF variant was present as a polymorphism in the Baniwa. These data have recently been incorporated into a treatment which concludes that the eight electrophoretically-defined “private polymorphisms” thus far encountered in Amerindian tribes can be explained by a mutation pressure of 0.7 × 10−5/locus/generation on the assumption of neutrality of the phenotypes in question (Neel and Thompson, '78).