A radioimmunoassay was developed which enables accurate determination of haloperidol [1-[3-(4-fluorobenzoyl)propyl]-4-hydroxy-4-(4-chlorophenyl)piperidine, a neuroleptic drug used in the treatment of psychoses] in human serum. Antiserum was prepared by immunizing guinea pigs with haloperidol (O-carboxymethyl)oxime derivative coupled with bovine serum albumin. With the antiserum, 3H-haloperidol and dextran-coated charcoal, the assay of haloperidol in serum was possible over a concentration range of 1-50 ng/ml, using 0.1 ml of human serum without the need of an extraction procedure. Data obtained by radioimmunoassay agree with those obtained by gas chromatography. No appreciable crossreactivity was observed with haloperidol metabolites or with other butyrophenone neuroleptics. Serum levels of haloperidol in schizophrenic patients receiving single oral dosing (6 mg/subject) were determined.