An improved procedure for the sensitive determination of the neuroleptic drug, haloperidol, in human serum is described. The method is based on a single hexane extraction procedure and a radioimmunoassay. Antiserum was elicited in guinea pigs immunized with haloperidol hemisuccinate derivative coupled to bovine serum albumin. Any appreciable cross-reactivity was observed neither with known metabolites of haloperidol nor with haloperidol decanoate (KD-136) . Some devices in the extraction procedure, e.g. the use of 10 ml of extraction solvent, made it possible to measure haloperidol levels as low as 0.05 ng/ml. The accuracy and reproducibility of the method were shown to be sufficient. The present method can be used to study the pharmacokinetics of haloperidol in the phase 1 study of KD-136. In addition, it is simple enough for use in clinical laboratories that are monitoring haloperidol concentrations in the blood of psychiatric patients.