SummaryThe effect of 2000 r of 200 kvp x-rays on the acid and alkaline-phosphatase content of mouse tibiae up to 80 weeks after irradiation is described. The enzymes were demonstrated histochemically using the coupling azo dye method. The alkaline phosphatase content was diminished within 24 hours after irradiation, due mainly to the loss of mesenchymal cells from the metaphysis and the disorganization of the epiphyseal plate. The amount of enzyme present further diminished with time compared with controls, except where nests of apparently normal cartilage cells appeared in the epiphyseal plate. These eventually gave rise to a new epiphyseal plate which was relatively normal in structure and enzyme content. The articular cartilage cells contained apparently normal amounts of alkaline phosphatase throughout the period of observation and the periosteum and endosteum made good recovery.The acid phosphatase content was not so markedly affected by irradiation. After an initial transient increase there was a reduction in the acid phosphatase content compared with the control except in tibiae where a new epiphyseal plate was being formed or in areas of necrotic bone where there were increased amounts of enzyme.