Rabbit antibody was elicited against purified oat phytochrome polypeptides. Immunodiffusion and immunoelectrophoresis indicated the antibody elicited was predominantly a single precipitin system. No antigenic difference was detected between red-absorbing phytochrome and far red-absorbing phytochrome. Crude preparations of rye and corn phytochrome showed a line of identity when cross-reacted with oat polypeptide phytochrome; pea phytochrome showed a line of partial identity. Precipitin reactions with purified rye phytochrome analyzed with sucrose density gradient centrifugation and immunodiffusion confirmed that the same class of determinants was available to the antibody when the protein was known to be in a state which had not undergone extensive proteolytic attack.