SV7, a progeny of Moloney murine sarcoma virus 349 cells, was molecularly cloned. SV7 induced sarcomas consisting of vascular and fibrous components. The large blood-filled vascular dilatations appeared grossly as dark red spots in the tumors and constituted up to 50% of the tumor volume. These vascular structures, ranging from small capillaries to cavernous vascular dilatations, were lined by one to several layers of neoplastic endothelial cells. Thick papillary outgrowths of the neoplastic endothelium extended into and often occluded the vessel lumens. The fibrous component consisted mostly of spindle cells and granulocytes, which provided the stroma for the vascular structures. The vascular and fibrous components appeared to have arisen independently. Lymphopenia accompanied by myeloid metaplasia was observed in the spleen of both SV7- and myeloproliferative sarcoma virus (MPSV)-infected mice. The blood of SV7-infected mice had a much higher level of circulating granulocytes than did that of MPSV-infected mice. The latter manifested a more advanced myeloid metaplasia, characterized by aggregates of myelomonocytic blast cells in the spleen.