Rabbit, primate, and human exposures were made to ultraviolet radiation in 10 nm wavebands to establish the photokeratitis thresholds. The limits of the rabbit and primate abiotic action spectrum were found to be from 210 nm to 320 nm. The peak of the UV threshold curve was found at 270 nm for both the rabbit and primate. The rabbit threshold at 270 nm was 0.05 × 106 ergs/cm2. The human threshold was 0.04 × 106 ergs/cm2 at 270 nm. Formulas for calculating protection against ultraviolet are presented.