Plasma level of beta-thromboglobulin (BTG) and platelet factor 4 (PF4), two platelet-specific proteins, were measured in 20 patients with cancer. None of the patients had evidence of thromboembolism or impaired renal function, and none was taking drugs known to interfere with platelet function. In ten patients who were in complete remission or whose disease was thought to be inactive, the levels of both proteins were essentially within normal limits. The other ten patients had active or progressive symptomatic disease. BTG level was elevated in all but one patient, and PF4 was raised in two patients. These results suggest that in patients with clinically active cancer, a state of ongoing in vivo platelet activation may be present, a phenomenon which may contribute to the known increased occurrence of thromboembolism in patients with malignancy. The intravenous administration of various cytotoxic drugs used in cancer chemotherapy produced no immediate measurable changes in BTG and PF4 level.