An antiserum against cabergoline, a powerful dopamine-agonist under clinical trials for the treatment of Parkinson's disease and hyperprolactinemia, has been raised in rabbits by immunization with an immunogen produced by conjugation of cabergoline to bovine serum albumin. The antiserum was able to bind a derivative of cabergoline labelled with tritium and was able to distinguish the drug molecule from some of its close related compounds and from other agents that could be simultaneously present in plasma from patients undergoing treatment with cabergoline. The antiserum and the tritium labelled hapten were used to develop a radioimmunoassay for cabergoline determination in human plasma and urine. A linear relationship between cabergoline added and % radioactivity bound was found in the range 1.9-500 pg/tube. The addition in the assay of 200 microliters human plasma or 25 microliters urine did not affect the specific and the non-specific binding of the radiolabelled hapten so enabling us to obtain a final sensitivity of about 12 pg/ml plasma and 120 pg/ml urine. The assay was validated in terms of reproducibility, precision and accuracy over the whole range of concentrations tested both in plasma and urine. The plasma concentrations at the steady state in a patient with Parkinson's disease who had received the drug at single oral daily doses of 3, 5 and 7 mg were determined using the assay.