Blood samples from members of the Oromo and Amhara ethnic groups of central Ethiopia were tested for 10 erythrocyte protein systems: ACP1, ADA, AK1, CA2, ESD, G6PD, GLO1, HBβ, PGD, and PGM1. Differences between the two samples were relatively slight and not statistically significant. Gene frequency distributions were then analyzed in the context of the genetics of the African and Arabian peoples. Considering the erythrocyte enzyme data, the Oromo and Amhara appear quite similar to Europoids (particularly to the South Arabians) and considerably different from the Negritic peoples. There is evidence for close genetic affinity among the Cushitic- and Semitic-speaking population groups of the Horn. Admixture between Europoid and Negritic populations seems to have been the main microevolutionary factor in generating the present day Cushitic (and Semitic)-speaking group of eastern Africa. The results are consistent with the hypothesis, supported by historical and linguistic evidence, for a common origin of these groups from a Cushitic-speaking group living in eastern Africa. © 1996 Wiley-Liss, Inc.