An electron microscopic study was performed on a conjunctival squamous cell carcinoma in an 82-year-old man. Biopsies were taken before and during beta-irradiation and the morphologic features of nonirradiated tumor cells were compared to those after application of 10,000 rad. There occur striking changes in the nucleus and the cytoplasm after irradiation. The nuclei show hypertrophic and electron-dense nucleoli as well as altered chromatin structures. The cytoplasm of many cells appears destroyed and contains numerous vacuole-like structures, as well as swollen mitochondria. It is of interest that numerous normal lymphoid cells were present within the irradiated tumor.