A high performance thin layer chromatographic assay for dapsone is described with a minimum level of detection of 20 ng ml-1 which is suitable for the study of dapsone pharmacokinetics in plasma and saliva. 100 mg dapsone was administered orally to seven normal adult volunteers, the mean plasma pharmacokinetic parameters were: alpha = 0.23 h-1; beta = 0.0236 h-1, and t1/2 beta = 30.2 h. Dapsone is also eliminated into the saliva and the t1/2 may be determined via its estimation in saliva. It is 73% bound to plasma protein and the saliva/plasma concentration ratio was found to be 27%. In two subjects the free plasma dapsone concentration was identical to the simultaneous salivary dapsone concentration. Therefore the salivary dapsone concentration is a measure of the free plasma fraction of dapsone. Saliva/plasma dapsone concentration ratios show no time or concentration dependence and little inter-individual variation but are unsuitable for acetylator phenotype determination because monoacetyldapsone is not eliminated in the saliva.