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Hairless mice with subcutaneously transplanted Lewis Lung carcinomas were used to study the effects of treatments with hematoporphyrin derivative (HPD) administered by intratumoral injection, followed by exposure to red light. A tumor cure rate of 70-90% was obtained following a single treatment. The therapeutic response increased with increasing HPD doses and light fluences. At a certain dose a plateau seemed to be reached, where further dose increase only gave a limited gain in treatment response. A narrow range seems to exist between beginning therapeutic response and frank necrotic destruction. The number of mice that died after treatment increased with increase in tumor size, and there was a correspondence between the time of death and the severity of the phototoxic reactions. The best therapeutic response was obtained when the irradiation was performed 1/2-1 h after injection, and with tumors less than 6-8 mm in diameter.