In blood samples from a Hindu population of Uttar Pradesh (North India) and from two Muslim groups, one from Andhra Pradesh (South India) and the other from Gujurat (West India), frequencies of 38 HLA-A, -B and -C antigens were investigated. Eight antigens – A23, A25, A29, A32, Bw45, B21, Bw22 and Bw53 – were absent in the Hindu population, four different antigens – A29, Bw52, B14 and Bw42 – were absent in Hyderabad Muslims, two antigens – A31 and Bw45 – were lacking in Surat Muslims. The three populations showed considerable genetic heterogeneity. The genetic difference between the two Muslim groups was small, but the Hindu population showed pronounced differences from each of the Mu