The effect of various treatments on the release of acid phosphatase from spleen homo-genates of irradiated rats was studied. Enzyme release increased 3 hours and 24 hours after a single dose of 1000 r of whole-body X-irradiation. Spleen homogenate irradiated in vitro did not show increased acid phosphatase release. Rats injected with thyrotropic hormone or spleen homogenate treated with L-thyroxine in vitro showed an increased enzyme release similar to that of X-irradiation. Thus, the X-ray effect on this enzyme release cannot be attributed to a direct action on membranes of acid phosphatase-containing particles of the cells but is apparently mediated through a hormonal mechanism.