Antisera to buprenorphine were obtained in rabbits immunised with 3-0-carboxymethylbuprenorphine and N-hemisuccinyl-norbuprenorphine conjugated to obvine serum albumin. Using the latter antiserum and tritium labelled buprenorphine a radioimmunoassay having good accuracy and precision was developed for concentrations as low as 50 picograms in 1 ml of plasma. The N-hemisuccinyl antiserum crossreacted with norbuprenorphine, and the 3-0-glucuronide conjugate with the 3-0-carboxymethyl antiserum. Cross-reactivity of both antisera to other pharmacologically related compounds was negligible. The assay was employed to determine plasma buprenorphine concentration following its parenteral administration to dog and man.