Sultosilic acid piperazine salt (A-585), a new lipid lowering drug, was compared with the action of bezafibrate. In 20 patients suffering from primary hyperlipoproteinemia a double-blind cross-over study was carried out. A series of parameters of the lipoprotein metabolism, as well as of fibrinolysis and platelet function, was compared before and after administration of the drugs. There was a statistically significant diminution of total cholesterol, triglycerides, beta- and pre-beta-cholesterol and an increase of alpha-cholesterol by both drugs. Furthermore, both drugs caused a significant shortening of the euglobulin lysis time and a significant diminution of platelet adhesiveness. In patients who were under oral anticoagulants the thrombotest levels were not influenced by A-585 but were frequently below the therapeutic range during bezafibrate therapy. The difference in the thrombotest during treatment with the 2 drugs was significant. A slightly elevated gamma-GT activity normalized during bezafibrate therapy but was not influenced by A-585. Other parameters did not show significant differences between the 2 drugs.