Immunological aspects of barley stripe mosaic virus protein (BSMVp) polymerization studies are reported here and in the accompanying papers. Immunodiffusion and immunoelectrophoretic analyses of different BSMVp intermediates (10 S and 20 S aggregates, etc.) and intact virus were carried out. Anti BSMV (As BSMV) serum can be entirely exhausted of its activity by absorption with different BSMVp aggregates beginning at the level of the 10 S aggregate. The amount of protein necessary to exhaust anti-BSMV serum (As BSMV) depends on the quaternary structure of antigen; a considerably smaller amount of 10 S protein than of BSMV was required to exhaust As BSMV. The exhausting dose of monomeric protein for its specific antibodies is very low, but after absorption the As BSMV still reacted with 10 S, 20 S, and BSMV. BSMV-anti BSMV" complex was examined in the electron microscope. The mode of arrangement of antibody molecules on the surface of BSMV particle is discussed. © 1968."