Relation of tumor-associated macrophages to tumor neovascularization in murine transplanted tumor, sarcoma 180(S180), was quantitatively analysed in situ on days 3, 6 and 9 after tumor implantation. Infiltrating macrophages in tumor were identified in paraffin section by alpha-naphthyl butyrate esterase stain and subdivided by electron microscopy according to the peroxidase activity. Simultaneously, the structure of newly formed microvessels was quantitated under light microscope by stereological morphometry. The results showed that, as the tumor grew and blood vessels proliferated, the number of infiltrating macrophages in tumor increased obviously but the ratio of peroxidase (PO) positive mononuclear phagocytes tended to decrease in contrast to the increase of PO-negative phagocytes. Microvasculature volume was found to be positively correlated to the proportion of PO-negative macrophages but negatively correlated to the proportion of PO-positive mononuclear phagocytes. It was indicated that tumor angiogenesis was related to the mature macrophages in the tumor.