Following total-body irradiation of young female CAF1 mice, fresh or dry splenic weight declined to a minimum in 1-5 days, depending upon the dose (30-800 r). Part of the splenic weight loss was secondary to diminished food intake. At doses below 400 r, recovery began prior to day 5. When the day-5 relative weight was treated as a linear function of log X-ray dose, 2 of 5 experiments showed significant deviations from linearity. The pooled minimum weight as a function of dose was approximately equal to_A + B, where_A = 52-2 e .00845 D B_= 48.2 e -.00139 D, and D = dose in roentgens. The ED50 (minimum weight) of 177 r was lower than the ED50 (day-5 weight) of 275 r because the latter involved recovery. The use of weight loss in the testis, thymus, and spleen as a measure of radiation effect was reviewed critically. It was concluded that the testis can yield the most reliable data.