Peculiar mononuclear cells existing in the mesenchymal stroma of human chorionic villi, termed Hofbauer cells, are macrophages, and they are characterized by many vacuoles of various sizes and small granules in the cytoplasm. Their functional properties are still a matter of controversy. The current study was designed to determine the biological association between these macrophages and human chorionic gonadotropin (hCG), mainly using immunohistochemistry and immunoelectron microscopy. By light microscopic immunohistochemistry, these macrophages showed a weak positive reaction for the hCG-alpha subunit and a marked positive reaction for hCG-beta subunit (hCG-β). They were observed throughout gestation, and the reactivity was unrelated to the fetal sex. However, immunoreactivity for hCG-β C-terminal peptide was negative. Immunoreaction products of hCG-β were localized in coated pits, coated vesicles, multivesicular bodies, and also within the cytoplasmic vacuoles and granules but not in the intracytoplasmic spaces, organelles, or nucleus. In addition, acid phosphatase activity was observed in these vacuoles and granules. The hCG/LH receptor was not defined in the cell suspensions of these macrophages by hCG radioreceptor assay. The present study provides evidence that these macrophages ingest hCG secreted from the syncytiotrophoblasts without a receptormediated endocytotic mechanism, and they may participate in the control of hCG. Both cell types are not mutually exclusive in the developing chorionic villi.