The development of a specific high-pressure liquid chromatographic (HPLC) assay, having a quantitative detection limit of 0.4 microgram/ml, is described for plasma indocyanine green. In a preliminary study, the HPLC method demonstrated that te traditional spectrophotometric procedure inaccurately quantitates the dye's degradation in aqueous medium since degradation products that absorb light at 770- nm are formed. In a further study, the spectrophotometric method yielded an erroneously low clearance of indocyanine green in the rabbit. In addition, the HPLC assay points to the possibility of metabolite formation of the dye in vivo.