Monocyte dysfunction has been reported in patients with cancer. The generation of a procoagulant tissue factor is a marker of the monocyte's activation. Since the only blood cell capable of generating tissue factor is the monocyte, the incubation of citrated blood with either saline (control) or endotoxin (monocyte activator) followed by determination of the recalcification time should yield a measure of monocyte activation. The recalcification times of the saline-incubated samples were similar for healthy women and those with cystic hyperplasia, but were significantly shortened in patients with breast cancer. The recalcification times of the endotoxin-activated samples were significantly shorter in patients with breast cancer than in those with cystic hyperplasia, which in turn was significantly shorter than in healthy women.