Generation of procoagulant activity was studied in leukocytes and lymphocytes from normal human peripheral blood, leukocytes from patients with chronic myeloid leukemia (CML), chronic lymphatic leukemia (CLL), rabbit peripheral leukocytes, and macrophages harvested from the peritoneal cavity of rabbits. Marked procoagulant activity developed in peritoneal macrophages and rabbit peripheral leukocytes when incubated with endotoxin. Human peripheral blood leukocytes and lymphocytes also generated procoagulant activity. When endotoxin was omitted the procoagulant activity generated was considerably (3-12 times) lower. Leukocytes from patients with CML were less active than normal leukocytes. Leukocytes from patients with CLL generated very little procoagulant activity even after prolonged incubation. An antibody to rabbit tissue factor neutralized the tissue factor activity generated by rabbit leukocytes.