Hemagglutinins of adencvirus Type 3, 4 and 7 were found to be separable from soluble complement fixation (CF) antigen and infectious virus. Inhibition of hemagglutination (HA) activity with rabbit serum antibodies appeared to be type specific. The HA component was relatively stable to heat, formalin and storage at 4[degree]C. It was more readily filterable than CF antigen and was produced earlier than the CF in infected monkey kidney cells.