The effects of prostacyclin (PGI2) on postoperative blood loss and on the deposits which are known to form on filters in the bypass circuit were studied in patients undergoing operations with cardioopulmonary bypass. In this double-blind, randomized study, PGI2 or a placebo solution was administered to 56 patients undergoing elective cardiac operations. At the end of cardipulmonary bypass, the arterial line filter was removed from the circuit and blood losses were accurately recorded. There was no statistically significant difference in either the amount or the pattern of postbypass bleeding between the PGI2 and the control patients. However, the changes in weight of the arterial line filters and their electron microscopic appearances suggest that PGI2 can reduce the deposition of platelets and fibrin on the filter mesh, and in this role it may be of value in reducing visceral injury during cardiac operations.