1 A new sensitive and rapid radioimmunoassay procedure for the determination of the plasma concentrations of the neuroleptic drug trifluoperazine is described. 2 The antiserum developed for trifluoperazine cross-reacted with N-desmethyltrifluoperazine and 7-hydroxytrifluoperazine to the extent of 26 and 24% respectively but its cross-reactivity with commonly co-administered tricyclic antidepressants and antianxiety agents tested was negligible. 3 The assay, based on the above antiserum, enabled the quantitation of 50 pg of the drug in 200 microliters of plasma with a coefficient of variation of about 2% and therefore should be applicable for singly dose pharmacokinetic and bioavailability studies. It should be applicable to therapeutic monitoring of the drug in patients.