It was shown by gel chromatography, sucrose gradient centrifugation and immunodiffusion that tobacco rattle virus (TRV) coat protein can exist in various discrete states of aggregation. MW values of about 20,000, 75,000 and 210,000 were estimated for the coat protein subunit and 2 discrete oligomeric aggregates, and a sedimentation coefficient of about 35S for the characteristic disc-like polymeric aggregate. Some serological differences between the 35S-aggregate and a mixture of oligomers and monomers of the coat protein subunit were demonstrated by agar gel immunodiffusion and immunoelectrophoresis.