Relapsing fever caused by Borrelia spirochetes is characterized by episodes of spirochetemia, fever, and DIC. We examined the ability of Borrelia hermsii to induce production of leukocytic pyrogen and thromboplastin from human blood leukocytes in vitro. Organisms were found devoid of endotoxin by the Limulus assay. Human peripheral blood leukocytes were separated into MNC and PMN fractions and were incubated with two to five spirochetes per cell in 10% human serum. Supernatant fluids from MNC-spirochete mixtures produced mean increases in the temperature of rabbits of 0.80 degree to 1.35 degrees C, which were significantly higher than those caused by supernatant fluids of MNC or spirochetes alone (p less than 0.05). MNC-spirochete mixtures possessed seven to 15 times the thromboplastic activity of MNC suspensions alone, assayed with a modified one-stage prothrombin time. Supernatant fluids of PMNs and spirochetes, on the other hand, did not contain leukocytic pyrogen, and PMN suspensions did not produce thromboplastin. Cycloheximide (10 micrograms/ml), and inhibitor of protein synthesis, completely suppressed both pyrogen and thromboplastin production. Although intracellular spirochetes were observed within phagosomes of blood monocytes by electron microscopy, the production of leukocytic pyrogen and thromboplastin was not significantly altered by serum opsonins or by the inhibitors of phagocytosis cytochalasin B (5 micrograms/ml) or phenylbutazone (2 mg/ml). These results showed that Borrelia spirochetes stimulated human MNCs to produce increased amounts of leukocytic pyrogen and thromboplastin and that this stimulation required de novo synthesis of protein, was not mediated by endotoxin, and was not prevented by omitting opsonic proteins or by inhibiting phagocytosis.