A radioimmunoassay has been developed which makes possible sensitive and specific determination of an active metabolite of cadralazine, (±)-6-[ethyl (2-hydroxypropyl) amino]-3-hydrazinopyridazine (ISF-2405), in plasma and tissues. On account of its lability in a sample solution, ISF-2405 in biological samples was selectively derivatized to a stable form, (±)-6-[ethyl (2-hydroxypropyl) amino]-3-(3, 5-dimethyl-1-pyrazolyl) pyridazine (ISF-3349), by treatment with acetylacetone. The concentration of ISF-2405 was determined by the assay of the resultant ISF-3349. Antiserum against ISF-3349 was elicited in guinea pigs immunized with the ISF-3349 derivative coupled with bovine serum albumin. The obtained antiserum was highly specific for ISF-3349, and did not cross-react with either cadralazine or its metabolites. [Pyrazole-4-³H] labeled ISF-3349 with a specific activity of 13.9 Ci/mmol was used as the radioligand. Assays of ISF-2405 in plasma and tissues were possible over the concentration range from 0.4 to 6.4 ng/ml with 1 ml of plasma, and from 2 to 64 ng/g with 100 mg of tissue. Plasma and blood vessel levels of ISF-2405 in rats after single oral administration of cadralazine have also been determined by the present method.