Macrophages secrete a variety of biologically active substances into their local milieu, including proteins, lipids, nucleotide metabolites, and oxygen metabolites. To date, more than 50 substances secreted by macrophages have been reported: enzymes; enzyme inhibitors; plasma proteins such as complement components, coagulation factors, and apolipoprotein E; factors that regulate the functions of other cells such as interferon, interleukin 1, mitogens, and angiogenesis factor; and low molecular weight substances such as reactive metabolites of oxygen and derivatives of arachidonic acids. Macrophage-derived products are probably important in the local environment, and they are believed to be important in the physiological and pathological functions of macrophages in inflammation, tissue repair, lipoprotein metabolism, acute phase response, and in microbicidal, antiviral, tumoricidal, and immunoregulatory activities; however, macrophages may not be the sole source for the secretion of some of these products. The secretion of these products is intricately regulated, developmentally and environmentally.