A simplified and rapid method is described for phenotyping of sulfamethazine acetylation. Two hours after a test dose of sulfamethazine, free and total sulfamethazine are estimated in blood drawn either by capillary or venipuncture. The present method offers sharp segregation between slow and rapid acetylators, requires micro blood volumes drawn from finger- or ear pucture, and is more convenient for population genetics survey as well as for clinical investigations.