A radioimmunoassay for hydromorphone in human plasma was developed using a commercially available morphine-6-antiserum and tritiated dihydromorphine. In the assay, free and bound drug are separated using dextran-coated charcoal. The method quantitates hydromorphone in the 2.5-20-ng/ml range with minimum sensitivity at 1.0 ng/ml. Within-run precision for hydromorphone as shown by the standard error of the estimate of the linear regression equation is +/- 1.01 ng/ml. Between-run precision data for hydromorphone at the 2,5-, 10-, and 20-ng/ml levels gave percent relative standard deviations of 22.35, 10.96, and 8.55%, respectively. A plasma concentration-time curve from a subject administered a single oral dose of hydromorphone demonstrates the usefulness of the assay in monitoring drug levels in a bioavailability study. The method also is applicable to the analysis of hydrocodone in human plasma in the 10-80-ng/ml range with minimum sensitivity at 3.0 ng/ml. Within-run precision for hydrocodone as shown by the standard error of the estimate of the linear regression equation is +/- 1.06 ng/ml. Between-run precision data at the 15-, 30, and 60-ng/ml levels gave percent relative standard deviations of 12.48, 7.67, and 6.03%, respectively.