We determined acetylator phenotypes by means of a sulfadimidine test and found that 39% of the native population of the province of Jujuy, in northwestern Argentina, have slow acetylators. The frequency of the allele controlling slow acetylation was estimated at q = 0.65 +/- 0.001. This estimate differs from those previously recorded in native and Mestizo American populations. A review of the literature on acetylator phenotypes in the Americas revealed that it is not possible to discern any clear pattern of distribution in relation to the geographic variability observed.