About 80 Sephardim, 80 Ashkenazim and 100 Czechoslovaks were examined for the Y-specific RFLPs revealed by the probes p12f2 and p49a,f on TaqI DNA digests. The aim of the study was to investigate the origin of the Ashkenazi gene pool through the analysis of markers which, having an exclusively holoandric transmission, are useful to estimate paternal gene flow. The comparison of the two groups of Jews with each other and with Czechoslovaks (which have been taken as a representative source of foreign Y-chromosomes for Ashkenazim) shows a great similarity between Sephardim and Ashkenazim who are very different from Czechoslovaks. On the other hand both groups of Jews appear to be closely related to Lebanese. A preliminary evaluation suggests that the contribution of foreign males to the Ashkenazi gene pool has been very low (1% or less per generation).