Various conventions have been used to express the activity of polymorphic hepatic N-acetyltransferase of isoniazid. Among them, two of the most common are the "percentage of acetylisoniazid" and the "inactivation index." A third alternative convention is proposed, the "molar acetylation ratio," which showed the most clear-cut bimodal distribution when applied to the results obtained for Cuna and Teribe Amerindians living in Panama. Through this method a unique antimode was assigned to each Amerindian group and the same frequency of slow acetylators (24% to 29%) was found, unlike the results obtained by conventional approaches and antimodes derived from white populations.