Factors V, VII, VIII, X, XI, and XII of the coagulation system, platelet count, and antithrombin III levels were assayed in 20 healthy volunteers aged 20-40 years and 61 elderly subjects aged 66-96 years whose skinfold thickness was also measured. Factors XI, XII, and antithrombin III levels tended to increase in women and decrease in men while factors X, VII, and V tended to increase in both males and females with advancing years. No age or sex differences were found in platelet counts or factor VIII levels. Factor VIII levels were inversely correlated with obesity in elderly males (r = -0.56, p < 0.005).