We describe a radioimmunoassay for the neuroleptic drug flupenthixol, suitable for routine monitoring of its concentration in blood. Antibodies for the assay were raised in a sheep against a 7-carboxyflupenthixol/ovalbumin conjugate. The resulting assay, with [3H] flupenthixol as the label, is capable of detecting 2.0 microgram of flupenthixol per liter, in a 100-microliter plasma sample. The antiserum shows no cross reactivity with tricyclic drugs and low interference from the major metabolites of flupenthixol. Concentrations in plasma after a single oral dose of flupenthixol have been followed in one volunteer. Peak values were reached after 3 h. Determinations of flupenthixol after fortnightly intramuscular depot injections of the sustained-release preparation, flupenthixol decanoate, showed the extent of fluctuations during this period.