Both the heat labile flagellar or H antigens and the heat stable somatic or O antigens of Clostridium tetani take part in complement fixation reactions. Type specific reactions due to the flagellar antigens are given in high titer by types I, III, VII, and VIII. Type VI strains are non-flagellated and devoid of type specific H antigen. Types II, IV, V, and IX, which possess an additional somatic antigen lacking in the other types, did not show type specific fixation but reacted strongly with each other. Cross-reactions in low titer due to the common somatic antigen occurred between all types.