A study of coagulation has been performed on 8 chronic renal failure patients receiving carbenicillin therapy. All showed a prolongation of the bleeding, recalcification, partially-activated thromboplastin, prothrombin and thrombin times. These findings suggest the presence of an anticoagulant with an heparin-like mode of action. In vitro tests suggest that carbenicillin may be this factor. We have also shown that the drug produces a disturbance in the normal polymerization process. The implications of these findings for the treatment of (CRF) patients with carbenicillin are discussed.