A low light level electronic imaging system is being developed to be used in a mass screening program for breast cancer detection in asymptomatic women. In this technique a mammographic image formed from an x-ray beam collimated by a scanning slit is converted to an intensified visible image which is viewed by a high resolution isocon television camera. The signal which is displayed on a small flat-faced cathode ray tube is photographed onto 70 mm roll film. Some advantages of the system for mass screening are its low cost of operation, ability to eliminate detection of scattered x-rays, short examination time, small format film images, low radiation exposure to the breast and the capability of electronic image processing. The limiting resolution of the system has been measured using lead test patterns and a phantom containing small diameter wires. In these experiments the electronic technique clearly demonstrated a 50u wire and barely detected a 25u wire. Extensive studies of the imaging properties of the system indicate that the image quality is limited chiefly by the television noise. In a clinical study, the ratio of the radiation exposure for the electronic method compared to that for the Lo-Dose film screen technique was measured at 0.17 + 0.06. At this level of exposure, the quality of the images was adequate to demonstrate cancers, calciTications, tumors, skin thickening, and trabecular structure. Calcification can be demonstrated, but at this Stage of development, very fine calcifications are not adequately visualized.