The long term goal of this project is the investigation of mechanisms of photosensitivity in patient who are abnormally sensitive to sunlight. A major aspect of the investigation is based on determining the action spectra; that is, identification of ultraviolet, visible or infrared radiation and the minimal dose of the radiation that will provoke changes in the skin. This is determined by irradiating the skin with narrow wave bands of measured intensity, obtained from high intensity xenon or xenon mercury lamps. Monochromatic light has been obtained with interference filters, but more recently a high intensity tandem prism - defraction grating mnochromator has been constructed which has been available for testing since January, 1969. A knowledge of the action spectra of a disease may lead to an understanding of its pathogenesis, or even biochemical identification of the photosensitizing substance. This hypothesis is based on the principle that radiant energy is only active after absorption, so that if the active wave length is known, a search can be made amongst substances which are known to absorb them. If the photosensitizing substance can be excited to produce autofluorescence, additional information can be obtained by adapting the defraction grating monochromator to form a microfluorospectrophotometer. The biochemical and biological research into the cause of photosensitivity includes derangements of porphyrin synthesis in human and experimental porphyrias; effect of ultraviolet radiation on DNA synthesis and more recently the investigator has observed that T lymphocytes are more ultraviolet sensitive than B lymphocytes. It is proposed to continue to study the practical application of T lymphocyte killing in various experimental models including inhibition of delayed hypersensitivity and graft versus host reactions.