In catabolism of fibrinogen a function of Cupfer endothelial cells isolated from dog and rabbit liver tissue, was studied. In lysosomal-mitochondrial fraction of Cupfer cells and activator of plasminogen was found. Proteolytic and esterase (hydrolysis of benzoyll-agronine methyl ester) activities were the most typical for lysosomal-mitochondril and supernatant fractions of Cupfer cells. The enzymatic hydrolysis of fibrinogen was not observed in cytoplasmic matrix. The data obtained suggest that under the physiological conditions non-coagulational catabolism of fibrinogen occurred mainly intracellularly.