Using clotting assay and radioimmunoassay (RIA), tissue factor activity (TFA) and TF related antigen (TFR:AG) were determined in an extracellular culture medium of HL-60 cells. After 12 h incubation, TFA and TFR:AG in the medium with endotoxin (EDX: 1 microgram/ml) reached maximums which were 1.8 and 2.1 times greater than those in the medium without EDX, respectively. In the leukemic cells of 10 patients with acute nonlymphoid leukemia (ANLL), TFR:AG showed a significant correlation with TFA (p less than 0.01). On day 1 of the induction chemotherapy, TFR:AG in the 7 patients with DIC significantly increased to 288.9 +/- 153.1 ng/ml (p less than 0.01), whereas no increase in TFR:AG was recognized in the 3 patients without DIC. These results suggest that TF may be released from leukemic cells into the culture medium or blood stream, and that this may correlate with the development of DIC.