RFLP analysis of some intra- and extra-genic polymorphic sites of Factor VIII (FVIII) and Factor IX (FIX) genes with relevant DNA probes or by polymerase chain reaction (PCR) was carried out in Slavic populations from the European part of Russia and also in the native ethnic groups of Uzbekistan and Kazahstan. The allele frequencies for the HindIII (intron 19) and XbaI (intron 22) polymorphic sites (PSs) in the FVIII gene were very similar in the two populations studied, but different for the intron 13 (CA)n repeat. Significant variations in the TaqI (intron d) and DdeI (intron a) polymorphisms of the FIX gene were evident between the Russian and Asian populations. Two unusual alleles (4.35 and 4.2 kb) for the extragenic PS St14/TaqI were registered in Slavs and one new allele (380 bp) for the DdeI polymorphic site of FIX was discovered in both Asian populations. Altogether, 210 haemophilia A (HA) and 24 haemophilia B (HB) families were subjected to molecular studies. So far, 160 HA and 12 HB families have been found to be informative for DNA analysis. Carrier status was ascertained in 42 HA and 6 HB female relatives, and rejected in 52 and 10 of them, respectively. The origin of some HA and HB mutations was traced with relevant polymorphic markers in several at-risk families. Prenatal diagnosis was accomplished in 28 HA and three HB families, resulting in the identification of 20 affected male fetuses.