A standardized, reproducible Ivy bleeding time technic has been described which permits one to obtain accurate bleeding time data in man. The technic was used to standardize an aspirin tolerance test in which 60 normal males had a control bleeding time; were given, on a double blind basis, either placebo or 1 Gm. of aspirin, and had a second bleeding time 2 hours later. The control values were: mean, 5 min.; mean ± 2 st. dev., 2 min., 30 sec. to 10 min. The values after placebo were: mean, 5 min., 30 sec.; mean ± 2 st. dev., 2 min., 30 sec. to 11 min. The values after aspirin were: mean, 9 min., 30 sec.; mean ± 2 st. dev., 4 min. to 21 min. The difference between the mean bleeding time after placebo and after aspirin was highly significant (p < 0.001). The distribution of the bleeding times after aspirin suggested that normal subjects do not respond to aspirin as a single population. The degree of prolongation of the bleeding time and the large size of the drops of blood observed in some subjects suggested to us that small amounts of aspirin may exert a significant effect upon hemostasis in normal individuals.