Microcolonies of 2-8 Madison-Darby canine kidney cells (MDCK II) and Chinese hamster lung fibroblasts (V79) cells were incubated with the photosensitizer Photofrin and exposed to light, and the resulting number of dead cells per colony was determined. The distribution of this number was found to be incompatible with the assumption that cells are inactivated independently. The experimental distributions were significantly different from the binomial distribution expected from this assumption, but in accordance with a model in which an inactivated cell can inactivate adjacent cells with a certain probability. These findings are contrary to the common view that damage caused by radiation is limited to the cell in which the primary damage takes place. Our findings clearly indicate some kind of cooperativity between cells treated with Photofrin and light.