Rabbits were immunized on six occasions during a seven month period with fluorocarbon and sucrose denstiy gradient purified preparations of human rhinovirus types 1A,2,3,4,9, OR 14. Sera collected 1 week after the final immunization formed 1 or 2 preciptin lines when reacted by immunodiffusion against fluorocarbon purified preparations of each homologous immunizing virus. Heterologous preciptin cross reactions were detected between: RV1A antigen and rabbit sera to RV2, RV9 and RV14; RV2 antigen and sera to RV1A; RV14 antigen and sera to RV3. The heterologous "group" or C-antigenic nature of the cross reactions was suggested by the merging of heterologous preciptin lines formed against rabbit sera with "group" antibody preciptin lines formed against human sera and by the location of heterologous reaction close to the antigen well. In addition, the presence of C-antigenic particles in the fluorocarbon treated RV2 preparation was suggested by the demonstration that a subpopulation of viral particles migrated to a pH of 4.4 by isoelectric focusing.