Thrombosis (infarction) frequency of rat tail after i.v. injection of kappa-carrageenin was significantly reduced during primary phase of adjuvant arthritis. In healthy rats thrombosis was not influenced by injection of soybean trypsin inhibitor or by strong local irritation after i.p. injection of lambda-carrageenin. Inhibition of thrombosis during adjuvant arthritis occurs contrary to the expectation because fibrinogen concentration and platelets count are increased and fibrinolysis is decreased during arthritis. Reduced thrombosis frequency in adjuvant arthritis might be due to inhibition of clotting processes and/or increased vasodilation.