Plaques were obtained via carotid endarterectomy and exposed to the fluorescent dye hematoporphyrin in vitro. Fluorescence emission spectra were used to characterize sites of dye binding. To provide additional information on the role of environment on fluorescence, model systems were studied involving micellar and premicellar systems. The data indicate two distinct fluorescence signals from plaque-bound porphyrin. These signals were at least 50-fold greater than the fluorescence from uninvolved regions of the vessel wall. Fluorescence emission at 618 nm was observed in all plaques examined. A second fluorescence signal at longer wavelengths was associated with the more complex plaques and could be mimicked by mixtures of porphyrin and detergent at a premicellar concentration. These studies suggest the presence of 2 different porphyrin-binding regions in the plaques examined. The first appears to be a predominantly aqueous site which may represent the interior of a cholesterol-rich micellar structure. The second is associated with 'hard' plaque, i.e., collagen and calcification.