Three spin labels were used to monitor possible drug-induced conformational changes in bovine serum albumin. The electron spin resonance spectra of labeled bovine serum albumin in all cases consisted of two subspectra corresponding to labels in a partially immobilized environment and a strongly immobilized environment. Only bovine serum albumin with the triazine spin label was suitable for quantitation of data. The strongly protein-bound acidic drug, phenylbutazone, caused a conversion of strongly immobilized sites to partially immobilized sites, an effect that was proportional to the amount of drug added. This probably was due to the initial drug binding inducing a conformational change in the bovine serum albumin, thereby exposing additional binding sites.