2 .l . Preparation of macrophage cultures Macrophages were obtained from hybrid C3DZF1 (C3H/Tifo9 X DBA/2 d) mice by peritoneal washing with phosphate-buffered saline without anticoagulant. Cells were routinely cultured in MEM (minimum essential medium with Earle’s salts, Gibco Bio-cult, Paisley, Scotland) supplemented with 0.05% (w/v) heat inactivated (lOO’C, 10 min) la&albumin hydrolysate (Nutritional Biochemicals Corp., Cleveland, OH) and 100 IU/ml of penicillin and streptomycin (Gibco Biocult). The cells were either seeded in Costar plastic plates (Costar, Broadway, Cambridge, MA) in 16 mm circular wells on glass coverslips (0.7 X lo6 cells seeded/well) or in 100 mm Falcon plastic dishes (20 X lo6 cells seeded/dish). After 2 h in culture non-adherent cells were washed away and new medium containing lactalbumin hydrolysate was added. The incubation took place at 37OC in an atmosphere of 5% CO2 in air. The medium collected after 24 h in culture was centrifuged at 500 X g and the supernatant tested for coagulation factors or subjected to chromatography on dextran sulphateSepharose for partial separation of the factors. In the experiments with [35S]methionine, cells were cultures in medium supplemented with 50 /.&i [35S]methionine/ml. After 24 h incubation the