The effect of different doses of sulphadimidine (250, 500, 750 and 1000 mg) on acetylation capacity in 10 normal individuals was investigated in a randomized cross-over study design. The subjects were initially phenotyped with an oral dose of 750 mg of sulphadimidine. Four weeks later, each subject was assigned four different doses of sulphadimidine. The acetylator phenotype was classified for each dose as rapid if the percentage of acetylated sulphadimidine in the post-dose 5-6 hour urine is more than 70% and as slow if less than 70%. The results indicate a clear separation into two phenotypes, even with the smallest dose of the drug. A slow acetylator in the 10 subjects showed a saturation of the acetylation capacity with increasing doses. This trend was not observed in the remaining subjects who were identified as rapid acetylators regardless of the doses. An oral dose of 750 mg of sulphadimidine is suggested as a 'standard' or optimum dose for acetylator phenotype testing.