A radioimmunoassay for chlorpromazine in plasma is described. The antiserum was obtained by immunizing rabbits with a conjugate of bovine serum albumin and N-(2-carboxyethyl)desmethylchlorpromazine. It is specific for chlorpromazine and its minor active metabolite, N-desmethylchlorpromazine. Other known active or inactive chlorpromazine metabolites and other psychotropic drugs tested do not cross react with the antiserum. Less than 34 pg of the drug can be detected in 200 muL of plasma. As many as 100 samples can be processed in a day by one technician. Concentrations of chlorpromazine can be measured in 200-muL samples of plasma collected as late as 48 h after a single oral 25-mg dose.