Assays for the potent, highly lipophilic analgesic buprenorphine described in the literature include the radioimmunoassay (RIA), radioreceptor assay (RRA) and selected ion-monitoring. Problems arise with the use of hazardous and unstable ligand in the RIA and RRA and the need for an extraction step for RRA and selected ion-monitoring. An enzyme immunoassay (EIA) was developed to allow rapid and simple analysis of plasma concentrations of drug without these disadvantages. The N-dealkylated derivative of buprenorphine, norbuprenorphine, was labelled with beta-D-galactosidase using the linking agent N-(gamma-maleimidobutyryloxy) succinimide. This enzyme-labelled hapten was used to develop an EIA for buprenorphine. The assay is sensitive enough to measure 10 pg of buprenorphine. Endogenous opioid peptides do not cross-react in the assay; norbuprenorphine itself was completely cross-reactive. Since the presence of plasma (100 microliters) has no effect on the standard curve, plasma levels in humans were determined by this EIA without extraction after intravenous or sublingual administration of the drug. The EIA therefore represents a good alternative to existing assays for buprenorphine-like material with the advantages of simplicity, safety, sensitivity and ease of handling.