THE development of a tumour can be taken as an expression of a disturbed equilibrium between the number of undifferentiated and differentiated cells of a tissue, in favour of the number of undifferentiated cells. Irrespective of the cause of this disorder, the derailed cells can show morphological, biochemical and immunological deviations. The purpose of this investigation was to compare the histochemical and enzyme histochemical pattern of cells of regenerating bone tissue and of cells of induced bone tumours. To determine the enzyme pattern of cells in regenerating bone tissue healing fractures have been studied. Several methods of inducing bone tumours are known: Radioactive compounds such as 45Ca, 89Sr, 90Sr, 239Pu and 226Ra (Anderson,