Whole blood procoagulant activity was determined by measuring the recalcification time of citrated blood, with and without the addition of bacterial endotoxin, in patients with breast cancer (n = 39), colorectal cancer (n = 20), benign breast disease (n = 15), benign colorectal disease (n = 11), normal volunteers (n = 15) and inpatients with non-malignant disease (n = 22). The median clotting times of those samples incubated with endotoxin were significantly shorter in the patients with breast and colorectal cancer compared with normal controls. Furthermore, significant differences between the median clotting times of stimulated and unstimulated samples within each subject group were observed only in the two cancer groups. There was no correlation between whole blood procoagulant activity and absolute monocyte counts, with histological staging or with plasma concentrations of plasma fibrinopeptide A. The results suggest that blood from patients with cancer is more sensitive to endotoxin stimulation than that from normal or benign controls, but that in its present form the technique cannot be used to distinguish between malignant and non-malignant disease.