The peritoneal fluid (PF) of women with infertility (especially in the presence of endometriosis) contains increased numbers of leukocytes, 90% to 95% of which are macrophages. The high numbers of peritoneal macrophages presumably result from an influx of blood monocytes into the peritoneum, and/or from local proliferation of peritoneal macrophages. Once in the peritoneal cavity, monocytes differentiate into tissue macrophages. Mononuclear phagocyte proliferation and differentiation are influenced by different cytokines, including macrophage colony-stimulating factor (M-CSF). The purpose of this study was to determine the relationship of M-CSF levels in human PF and plasma to the macrophage content, and to the patient diagnoses. Mean concentrations of PF M-CSF were higher than plasma levels (2.44 ± 0.13 v 0.95 ± 0.06 ng/mL, respectively). The mean concentrations of plasma M-CSF did not differ in samples from women of different diagnostic groups (normal, peritoneal adhesions, endometriosis, inactive pelvic inflammatory disease, uterine fibroids, and idiopathic infertility), but the PF concentration was slightly higher in normal women. The absolute (total) amount of PF M-CSF in normal women was lower than in those of the other diagnostic groups. The total amount of PF M-CSF in all women correlated closely with the total number of peritoneal macrophages. The tubal patency status (open versus closed) did not influence the plasma and PF concentrations of M-CSF, nor the PF absolute amount of M-CSF. The PF M-CSF may have come from peritoneal macrophages, fibroblasts, mesothelial cells, or endothelial cells. PF M-CSF may play important roles in the proliferation and/or the differentiation of peritoneal mononuclear phagocytes.