Inbred Balb/c mice bearing 2- or 4-week-old transplantable syngeneic sarcoma L-1, when injected intravenously with the cells of the same tumor, displayed a low or high number of tumor nodules in the lungs, two weeks after the injection. In order to test whether this phenomenon is dependent on changes in host immunity associated with tumor progression, the immunocompetence of spleen cells from tumor-bearing mice was measured in a lymphocyte-induced angiogenesis assay, performed on (C57Bl/Sn X Balb/c)F1 recipients. Splenocytes from mice bearing tumors of different age, whether treated or not with cyclophosphamide, showed, however, the same angiogeneic potency.