Plasma thromboxane B2 (TxB2) level were measured in patients with various disease, and its changes were investigated in patients who underwent various operations. The following results were obtained. Plasma TxB2 values in patients with TAO, heart disease, and malignant disease were significantly elevated compared with healthy control. Plasma TxB2 values during operation and immediate period after operation were significantly elevated compared with preoperative values. Plasma TxB2 level was markedly elevated in bypass operation when synthetic prosthesis were used. In patients with malignant disease, plasma TxB2 values significantly decreased after removal of tumor compared with preoperative values. In cardiopulmonary bypass, TxB2 level were markedly elevated ten minutes after beginning, and then decreased gradually until the end of cardiopulmonary bypass. The factors which are considered to be related to elevation of plasma TxB2 level during the operation and the postoperative period are as follows: a) surgical intervention, b) anesthesia, c) thromboembolic complication, d) grafting of synthetic prosthesis. e) valve replacement and f) cardiopulmonary bypass. The positive correlation between beta-TG and TxB2 level was found. TxB2 measurement is complex and too expensive, while measurement of beta-TG is very easy and cheap. Because of their good correlation, beta-TG is used as substitute for TxB2 in daily practice.