The plasma concentration of beta-thromboglobulin was serially measured in nine patients with septicemia, ten patients with pneumonia and five thrombo- and granulocytopenic patients with acute leukemia. Six patients with septicemia out of the eight studied on days 1-3 and all eight patients studied 7-14 days after onset had an abnormal high beta-thromboglobulin level. One patient with pneumonia out of six studied on days 1-3 and six out of nine studied on 7-14 days after onset had an abnormal high value. A rising trend in plasma beta-thromboglobulin with the highest mean levels at one to two weeks after onset was common to both groups. Positive ethanol gelation, increased level of fibrin/fibrinogen degradation products, decreased antithrombin III, increased FVIII complex and disproportionate ratio of FVIII:C to FVIIIR:Ag were common in both groups in the early stages of the disease. All the five patients with leukemia had a lower than normal beta-thromboglobulin level throughout the study but showed in the coagulation parameters changes similar to those observed in the other groups. Judging from the commonness of abnormal beta-thromboglobulin values in the two first patient groups, low grade platelet activation is a normal response in severe infection.