A spontaneous AP positive C3H murine OS was used to determine the effects of various treatment modalities. AP served as a useful circulating biomarker of the in vivo tumor growth. In animals whose tumor was amputated, the elevation of the marker indicated the presence of pulmonary metastases. It was used to establish the time of recurrence after a partially effective chemotherapy or combination modality. In this model, when the neoplasm was surgically excised at 10 days posttransplantation, 70--80% of the mice showed presence of lung metastases. Specific immunotherapy with irradiated tumor cells did not alter the course of the disease. Passively transferred immune allogeneic cells to tumor bearing mice at an effector to target cell ratio of 1:1 in vivo were ineffective. The murine OS model is extremely useful to plan and institute different combinations of treatment modalities and optimize the conditions for an effective treatment in a relatively short time.