Macaca mulatta monkeys were exposed to neutron and gamma radiation as one phase of an extensive animal program designed to yield a better under standing of the biological effects of ionizing radiation. Exposure distances were chosen on the basis of dose vs. distance data obtained during Operation Teapot and on the desired biological responses. In both shots in which Project 39.6 participated, the radiation dose required to produce death in 30 days in 50 per cent of the animals was determined. All animals were followed for acute radiation effects, and, where no mortality occurred, they are being followed for long-term effects such as cataract production, bone-marrow change, shortening of life span, and carcinogenesis. Ultimately the long-term findings will be compared with known human data.