Tumor-associated macrophages (TAM) represent a population of tissue macrophages with peculiar biological, biochemical and phenotypic properties. Here we have briefly analyzed two different mechanisms involved in the regulation of the levels of TAM: the production of tumor-derived chemotactic factors for mononuclear phagocytes and in situ proliferation of TAM. Two clones selected from the murine sarcoma line B77 showed a different capacity to produce the tumor-derived chemotactic factor known as JE. Studies with these clones demonstrated a correlation between in vitro production of the protein JE, expression of JE mRNA and macrophage content in tumor tissues, suggesting that the production of chemotactic factors can play a role in the regulation of TAM accumulation. Moreover, it has been shown that TAM had high levels of proliferative activity compared to peritoneal exudate macrophages. In an effort to elucidate the mechanisms responsible for the proliferative activity of TAM, the expression of c-fms and macrophage-colony-stimulating factor (M-CSF) was investigated in TAM and sarcoma cells. TAM had high levels of mRNA transcripts of the c-fms protooncogene, which encodes a tyrosine kinase probably identical to the M-CSF receptor, but did not express M-CSF transcripts, while sarcoma cells had high levels of M-CSF mRNA. Sarcoma-cell-conditioned medium had M-CSF activity on bone marrow cells: this activity was blocked by anti-M-CSF antibodies. These findings outline a paracrine circuit in the regulation of TAM proliferation, involving M-CSF secreted by sarcoma cells and acting on c-fms-expressing TAM. A better understanding of the regulation and function of TAM may provide a less empirical basis for a rationale design of therapeutic approaches.