Immunoassay techniques offer promise for the selective analysis of enantiomeric substances without prior separation. In a demonstrationof this selective analysis, radioimmunoassays were developed for R- and S-warfarin. The 4'-carboxyethyl analog of R,S-warfarin was resolved and the enantiomeric acids were coupled to bovine serum albumin. Immunization of rabbits with the conjugates led to formation of antisera which selectively bound the predicted enantiomer. Cross-reactions with various metabolites of warfarin were low (0.1--4%) except for 4'-hydroxywarfarin, as were cross-reactions with the opposite enantiomer (0.3--3%). Assays were developed and used to determine enantiomer half-lives in rats dosed with racemic warfarin. Results were consistent with those found previously by administration of individual enantiomers, thus indicating the utility of the radioimmunoassay method.