Kulonen, E. & Potila, M. Peritoneal macrophages and the synthesis of connective tissue components. Acta path. microbiol. scand. Sect. C. 88: 7-1 3, 1980. I . Macrophage homogenate, added to the incubation medium of granulation tissue slices, decreases the incorporation of labelled proline to collagen and other proteins, to a greater extent in the presence of certain prostaglandins and in oxygen atmosphere, but to a lesser degree if serotonin and cyclic AMP are added. 2. Rheumatoid synovial fluid, connective tissue active peptide (CTAP) of Castor and an extract from CClcdamaged liver stimulate collagen synthesis in the presence of macrophages. 3. Preparations from non-treated macrophages stimulate the incorporation of glucosamine to acid mucopolysaccharides, mainiy hyaluronate. 4. In a developing rat granuloma, the formation of collagen was stimulated by homogenized macrophages and by macrophage culture medium, but especially by whole macrophages in the presence of Si02.This study was inspired by the finding that SiOz releases from macrophages a nfibrogenic factor({ ' which stimulates protein synthesis in incubated granulation-tissue slices (1) and in cell-free polyso-ma1 systems (4). Since Si02 and related minerals