hrombospondin, one of the major glycoproteins released from alpha- granules of thrombin-stimulated platelets, is a disulfide-linked trimer of 160,000-dalton subunits. Cultured human monocytes secreted thrombospondin (determined by an enzyme-linked immunosorbent assay) into the culture medium in a time-dependent manner (1.45 micrograms/10(6) cells/24 hr); secretion was totally blocked by cycloheximide (1 microgram/mL). 35S-thrombospondin was isolated from 35S-methionine-labeled human monocyte postculture medium with rabbit polyclonal anti-thrombospondin coupled to protein A-Sepharose. The immunoisolated 35S-thrombospondin migrated in sodium dodecyl sulfate- polyacrylamide gels after reduction with a molecular weight of 159,000. Similar results were obtained using mouse resident peritoneal macrophages. Elicited peritoneal macrophages harvested from mice pretreated with endotoxin, casein, or thioglycollate secreted much less thrombospondin than did resident macrophages harvested from control mice. Thus, monocytes and macrophages from two different species synthesize and secrete thrombospondin, and the rate of synthesis of thrombospondin appears to depend on the state of activation of the cells.