An attempt was made to evaluate a biological effect to the CNS and an antitumor effect in vivo and in vitro by argon laser photoirradiation associated with or without a fluorescein durg. The injury to the CNS was evaluated dose-dependently by exposure of the mice brains, and the photoactivated effect was evaluated by exposure of the spinal cords of the cats after cooperation of fluorescein. Retardation or temporary arrest in growth of a mouse fibrosarcoma (Fsa II) implanted subcutaneously in C3H mouse, was accomplished by exposure of the tumor to activating argon laser after cooperation of fluorescein. The photoirradiation therapy of argon laser proved lethal dose-dependently to the glioma cells in culture. The preliminary and basic study suggest a new method to the treatment of human gliomas by using an argon laser photoirradiation with fluorescein drug. However, a thermal effect to the CNS of the argon laser photoirradiation is very extensive, and further development and research are planned.