Results of protease inhibitor (PI) subtyping on polyacrylamide gel isoelectrofocusing of 599 Israeli non-Jews and 1,393 Israeli Jews are recorded. A discriminant analysis (DS) was performed on frequency data of the 5 PI alleles (M1, M2, M3, S and Z) with data of Europeans, Israeli non-Jews and Israeli Jews. A higher percentage of correct classification was obtained when Jews were treated as a separate population group rather than when distributed in their areas of origin. This suggests a greater resemblance, in the PI system, of the studied Jewish groups to each other than of the European Jews to Europeans and of the studied mediterranean Jews to Middle Eastern non-Jews. A cluster analysis disclosed distance relationships in a similar direction. PI allele distribution, in the studied Jewish samples, has the following characteristics: Jews share with Middle Eastern non-Jews an absence of PIZ, which is present in Europeans. Mediterranean Jews have higher frequencies than Ashkenazi Jews, of PIS alleles, which are absent in Middle Eastern non-Jews. European Jews are closer to the Europeans than Middle Eastern Jews in their PIM allele frequencies. An original common gene pool of Jews with Middle Eastern non-Jews is postulated, of which the Sephardic (Spanish) and Middle Eastern Jews differ, now, in having PIS, and European Jews differ in having slightly lower PIM3 and PIM2 and higher PIM1 frequencies. A possibility of admixture and selection, affecting different alleles in different Jewish communities at different times, is suggested to have contributed to the present-day deviations from the supposed original gene pool.