Monocyte-enriched cultures were prepared from human blood mononuclear leukocytes by adherence to growth substratum. Synthesis and secretion of fibronectin was detected in these cultures concomitantly with morphological differentiation, starting on day 3--5. Production of fibronectin by macrophages was documented by metabolic labeling followed by immunoprecipitation and gel electrophoresis, radioimmunoassay specific for human fibronectin, and by indirect immunofluorescence. Fibronectin was detected mainly intracellularly but was also detected pericellularly only in minute amounts. No production of collagenous proteins was seen in these cultures. Macrophage fibronectin might act in vivo as a nonspecific opsonin and promote cell adhesion during macrophage migration in tissues.

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