Macrophages have been suggested to play a major role in the immune response to cancer. They have also been suggested to stimulate the formation of tumor stroma and to promote tumor vascularization. The availability of the op/op mouse, which has no endogenous colony-stimulating factor 1 (CSF-1) and which possesses a profound macrophage deficiency, provides a new model to verify these notions. Subcutaneous growth of transplantable Lewis lung cancer (LLC) is markedly impaired in the op/op mice compared with normal littermates. Treatment of tumor-bearing op/op mice with human recombinant CSF-1 corrects this impairment. Histological analysis of tumors grown in op/op and normal mice revealed marked differences. Tumors grown in op/op mice display a decreased mitotic index and pronounced necrosis, particularly hemorrhagic. Moreover, particularly in the op/op tumors, peculiar sinusoid-like abortive vessels (not filled with blood) have been observed. These tumors, in contrast to tumors grown in normal mice, are almost deprived of regular arteries and veins. In contrast to tumors grown in normal mice, they exhibit almost no Sirius red-stained collagenous fibers and Gomori silver-stained reticular fibers. Our data suggest that the CSF-1-dependent macrophage subpopulation missing in op/op mice plays a primary role in supporting tumor stroma formation and tumor vascularization in murine LLC tumors.