Abstract Antisera against reserpine have been developed in rabbits immunized with reserpine conjugated to bovine serum albumin. These antisera were used in a radioimmunoassay procedure for reserpine. No appreciable cross-reactivity was observed either with reserpine metabolites or with other drugs commonly used in conjunction with reserpine for the treatment of hypertension. The assay was able to detect as little as 150 pg of reserpine in serum (15 ng/ml), without the need for an extraction procedure. Serum concentrations were measured in rats following the oral and intravenous administration of 1 mg/kg of reserpine.