Serum blood samples from 563 of the total 700 Nganasans, members of the isolate in the northern-most part of Siberia were tested for G1m (z,a,x,f), G2m (n), G3m (g,b0,b1,b3,b5,s,t), and Km (1) allotypic determinants. Additionally, 78 Yenisey Samoyeds (Entsi) who are the Nganasan's western neighbours were studied. Both populations are remarkable for high frequency of 'Northern Oriental' Gm (za;..;b0b3b5st) which appears to be the most frequent haplotype in the Nganasans (0 . 486), and is the second frequent in Yenisey Samoyeds (0 . 276). The Gm (f;b) generalized haplotype which used to be considered as an indicator of Caucasian gene flow occurred in the Nganasans in the very low frequency of 0 . 008, versus 0 . 045 revealed in adjoining Yenisey Samoyeds. Both populations also differ in the frequency of Km1 which is two times lower in the Nganasans (0 . 048), than in Yenisey Samoyeds (0 . 103). When segregation ratios for the Gm locus were inspected in 67 Nganasan families, no apparent deviations from Mendelian expectations, and no recombinant phenotypes were observed.