The relative contribution of Europeans, Africans, and Amerindians to the gene pool of two Uruguayan populations (Montevideo and Tacuarembó) was estimated using several approaches. For Montevideo 8 genetic systems were considered, and for Tacuarembó 18 systems were used. A preliminary investigation of the most probable parental groups, using genetic distances, yielded four combinations of European populations, four combinations of African populations, and five combinations of Amerindian populations. Afterward, 240 possible combinations from the possible parental groups were considered for the quantitative estimations of interethnic admixture using the gene identity method. The most inclusive combinations furnished the following admixture estimates: (1) Montevideo, 92% European, 7% African, and 1% Amerindian; (2) Tacuarembó, 65% European, 15% African, and 20% Amerindian. The modal values obtained within each ethnic category did not differ by much (2-3%), the exception being the Amerindian contribution to Tacuarembó, where a higher diversity was observed (up to 14%). Comparison with a maximum-likelihood method of admixture estimation was hampered by the fact that not all markers can be used to obtain these alternative numbers. Evaluations using six systems for Montevideo and seven for Tacuarembó yielded values that were closer to the previous estimates for Montevideo (largest difference, 7% in the Amerindian component) but somewhat higher for Tacuarembó, amounting to 11% for the European and Amerindian contributions. It is clear, however, that the two populations show significant biological heterogeneity, resulting partly from diverse patterns of historical formation.