The fluorescence of endogenous porphyrins in tumors has been attributed to the presence of several different species of bacteria inhabiting ulcerated and necrotic tissues. We have examined this "autofluorescence" in carcinogen-induced tumors in the hamster oral mucosa and in patients with cancer of the oral cavity. In several instances, bright autofluorescence of normal tissues has been observed. This represents a potential false-positive identification of cancer when systemic hematoporphyrin derivative fluorescence is used as a diagnostic technique.