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Fialuridine (FIAU) is a halogen-substituted analog of thymidine that was undergoing clinical investigation as a drug for the treatment of chronic hepatitis B viral infection. However, clinical trials of FIAU were terminated after adverse events occurred following chronic oral administration. Prior to the termination of clinical trials, a sensitive assay was needed for the measurement of FIAU because of the anticipated low dose administered to patients. We therefore undertook the development of a radioimmunoassay (RIA). A specific antiserum was raised in rabbits following immunization with a 5'-O-hemisuccinate analog of FIAU coupled to keyhole limpet hemocyanin. Radiolabeled FIAU was synthesized by a destannylation procedure by using sodium [125I]iodide. We developed a competitive-binding procedure and used precipitation with polyethylene glycol as the method for separating the bound and free forms of FIAU. The RIA is sensitive (0.2 ng/ml), specific (negligible interference from known metabolites and endogenous nucleosides), and reproducible (interassay coefficients of variation range from 5 to 19.7% for serum controls). We used the RIA to assess the pharmacokinetics of FIAU in healthy adult volunteers following administration of a single 5-mg oral dose. The sensitivity of the RIA permitted the detection of a prolonged elimination phase for FIAU in healthy volunteers and dogs, with mean elimination half-lives of 29.3 and 35.3 h, respectively. We conclude the RIA is a valid method for the quantification of FIAU in biological fluids.