A flying spot ultraviolet microscope, employing a fast scan and pulsed operation of the raster, has been used to induce radiation damage in ascites tumor slide cultures, and to study by time-lapse cinematography the progressive stages of cell damage. The cells observed came from a strain (EF7) of the Ehrlich ascites carcinoma. Irradiated cells were found to show a characteristic syndrome of damage, involving blebbing at the cell surface, while control cells in the adjacent areas of the preparation remained unchanged. The end of the blebbing period is marked by swelling of the cells, and the time taken for this phenomenon to occur was used as a measure of the severity of the damage. It was found that the time required for swelling is dependent on the size of the dose employed, as well as on the sensitivity of the cells. This latter sensitivity was found to decline as the physiological age of the tumor increased. If ultraviolet illumination below 255 mµ is excluded, no symptoms of damage occur, even when very large doses are used. These observations are discussed in relation to the nature of the system in the cell which is affected.

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