Histochemical staining for three hydrolytic enzymes were performed in 35 bone tumours and 43 soft tissue tumours, malignant as well as benign. Osteosarcoma, intra-osseous as well as extra-osseous, revealed characteristic rich staining for alkaline phosphatase, no matter how dedifferentiated the tumour was. Haemangioendothelioma (and normal endothelium), too, showed strong reaction for alkaline phosphatase whereas haemangiopericytoma did not. Alkaline phosphatase furthermore was found in slight to moderate amounts in fibrous proliferations. All other tumours examined were negative. Acid phosphatase was found in almost every tumour investigated except Ewing sarcoma and chondromyxoid fibroma. However, high activity was characteristic of giant cell tumours and malignant fibrous histiocytoma. The inhibition of acid phosphatase by tartrate was complete except in osteosarcoma and giant cell tumours, where only a partial inhibition was seen. There were non-specific esterase reactions in a variety of tumours, but very strong reactions were characteristic of malignant fibrous histiocytoma and giant cell tumours. The reaction could be completely inhibited by the addition of fluoride. In an era of increasing application of immunohistologic techniques in surgical pathology it might be of value to remember that simple enzyme histochemical stainings may provide helpful diagnostic features in the classification of bone and soft tissue tumours.