The concentrations of the plasminogen activators u-PA and t-PA as well as the inhibitor PAI-1 were studied immunohistochemically in 35 osteosarcoma specimens compared to 15 Ewing's sarcomas and various other bone lesions. The immunoreactivities of plasminogen activators in osteosarcoma specimens were significantly higher than PAI-1. Biochemically in cell cultures and immunohistochemically in the pathological specimens of the osteosarcomas u-PA was the dominant antigen, all factors showing a generally strong reactivity. Here a relation to the malignancy and invasivity of the tumors could be seen, which was comparable to the results in other tumors. This correlation, however, could not be detected in Ewing's sarcomas. On the whole low immunoreactivity of all enzymes, and of u-PA in particular, could be seen. In comparison in some benign lesions (fibrous dysplasias, aneurysmal bone cysts) the immunohistochemical reactions were distinctly stronger. Dominant antigens did not exist in Ewing's sarcomas. In all mesenchymal tumors studied the immunohistochemical localisation and concentration of the antigens showed a definite correlation with their histological differentiation. Chondroblastic differentiations in different tumors (chondrosarcomas, enchondromas, chondroblastic osteosarcomas) or chondroid parts were always negative for both activators and the inhibitor, especially in their central parts. Here, the biological malignancy was not decisive for the result.