We demonstrated that the in vitro differentiation of human peripheral blood monocytes to macrophages is dependent on the environment and conditions of monocyte culture. Cultivation of monocytes on glass or microexudate-coated glass gave rise to cells resembling foreign body granuloma macrophages. After an initial rise in Fc receptor- and C3 receptor-mediated phagocytosis, a progressive loss of Fc receptor expression and C3-mediated ingestion were observed. The monocyte surface antigens recognized by the anti-human monocyte monoclonal antibodies 1D5 and 63D3 were lost from the surface of the majority of cells cultured on glass and microexudates. A subpopulation of Fc receptor-positive cells that were 1D5 and 63D3 positive was retained in fully differentiated cell populations. In comparison, monocytes cultivated on collagen matrices gave rise to highly phagocytic cells resembling human resident tissue macrophages. Both Fc- and C3-mediated phagocytosis were enhanced and remained so during the entire length of culture. The surface antigens recognized by the 1D5 antibody, expressed on all freshly seeded monocytes, was maintained on the macrophages. The antigen recognized by the 63D3 antibody was not expressed on mature cells. The present evidence would indicate that variations in expression of phagocytic receptors and the surface antigens 1D5 and 63D3 can be ascribed to the stage of development of the macrophage or its stage of activation, rather than to independent subsets of mononuclear phagocytes.