Determinations of plasma euglobulin fibrinolytic activity were made in samples of plasma collected during one normal or hormone-simulated cycle in 15 young, normal women and in 11 women using an oral contraceptive low in estrogen. Distinct, individual levels caused the variations of fibrinolytic activity to be larger within the group than individually, and the activity showed only small changes during the cycle. The hormone group showed a marked increase in fibrinolytic activity with distinct fluctuations related to the phase of the cycle. This increase corresponded to that previously reported to be caused by oral contraceptives containing higher concentrations of estrogen.