An evaluation of the coagulation system has been conducted in vitamin E and/or selenium deficient swine. The partial thromboplastin time, plasma fibrinogen concentration, platelet lipid peroxides, as well as the fibrinogen/fibrin degradation products were not found to be significantly affected by either vitamin E deficiency, selenium deficiency, or deficiency of both. With selenium deficiency, the prothrombin time was shortened (p less than 0.05). The platelet count and platelet turnover were greatly decreased by both vitamin E (p less than 0.001) and selenium deficiency (p less than 0.005). Further-more, the survival of platelets labelled with 75Se-selenomethionine and the per cent isotope incorporated into platelets were reduced (p less than 0.05 and p less than 0.005) in association with vitamin E deficiency, but not with selenium deficiency. These results were interpreted as evidence of a platelet production defect and possibly a platelet function defect in vitamin E deficient animals. Selenium deficiency were also associated with decreased (p less than 0.05) survival of fibrinogen labelled with 75Se-selenomethionine and increased (p less than 0.05) turnover of fibrinogen. From these fibrinogen kinetic findings, it was considered that chronic low grade disseminated intravascular coagulation possibly occurs in selenium deficient animals, probably in relation to the development of hepatosis dietetica or widespread microvascular damage. However, other possibilities such as increased fibrinogenolysis in relation with hepatosis dietetica or an intrinsic fibrinogen defect due to selenium deficiency also need to be taken into consideration and have not been ruled out in the present study.