Groups of male Swiss albino mice were irradiated by single doses of either 7 rem or 14 rem of fast neutrons with 14 MeV average energy, corresponding to fluences of 1.27×108n/cm2and 2.54×108n/cm2, respectively. The activities of acid phosphatase (ACP) and succinic dehydrogenase (SDH) in kidney, lung and liver were determined at different time point up to seven days after irradiation. Lysosomal affection was respresented by statistically significant increase of ACP activity in all cell types of the three tested organs immediately after irradiation with either of the doses used. The effect of SDH was represented by reduction in activity in all three organs. The activities of both enzymes showed tendencies to return to pre-irradiation levels with time in most cell types especially after the 7 rem dose.