The number of alpha-granule membrane protein 140 (GMP-140) molecules was detected by using specific monoclonal antibodies (125I-SZ-51) against GMP-140 in three groups of patients with unstable angina treated with aspirin 50 mg (n = 27), 150 mg (n = 26) and 300 mg (n = 30) a day before and 7 days after treatment. The number of platelets was counted by using the routine method. The results indicated that the number of GMP-140 molecules decreased significantly and the number of platelets increased significantly after treatment with aspirin. With the increase of aspirin dosage the degree of these two changes increased (P < 0.01, both) too. When 300 mg of aspirin was used, the number of GMP-140 molecules was lower than that in a control group of healthy subjects (P < 0.005) and the number of platelets reached that of the control group (P > 0.05). There was no difference on the immediate side effects when these three dosages of aspirin were used. The authors of the opinion are that it is rational to use 300 mg of aspirin to treat patients with unstable angina at the beginning.