Human Rh+ and A+ red cells sensitized with human anti-Rh and anti-A blood group alloantibodies are shown to stimulate large increases of tissue factor activity as compared to controls after incubation with leukocytes for 3-18 hours. As little as a 1/1280 dilution of human anti-Rh serum was stimulatory. Immunospecific effects in experiments with 5 different antibodies exclude possible artifacts resulting from contaminants such as endotoxin. These results suggest that production of leukocyte tissue factor may be similarly stimulated by sensitized red cells during immune hemolysis and serve as an activator of coagulation.