The interaction of fibrinogen with monocytes was studied. After stimulation with ADP (10 microM) or thrombin (1 U/ml), platelet-free suspensions of human monocytes bind 125I-fibrinogen with two different affinities in a specific and Ca2+-dependent reaction with saturation at 5.80-7.35 X 10(-7) M of added protein. The binding of fibrinogen to specific receptors on monocytes induces the procoagulant activity of these cells. Thrombasthenic cells or normal monocytes preincubated with a monoclonal antibody to the platelet glycoprotein IIb/IIIa complex (10E5) do not bind fibrinogen and have no procoagulant activity. Metabolic studies with [35S]methionine revealed that cultured monocytes actually synthesize a surface antigen precipitated by 10E5 antibody as a major band with 92,000 relative molecular weight. Our data indicate that monocytes express receptors for fibrinogen only in part related to the platelet glycoprotein IIb/IIIa complex. Furthermore, the binding of fibrinogen to monocytes enhances the cooperation of these cells in hemostasis.