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Wheat streak mosaic virus capsid protein degraded in vivo by proteolysis as leaves senesced. The capsid protein of virus purified from young systemically infected leaves had an apparent size or 45 kDa in 12% sodium dodecyl sulfate-polyacrylamide gels with minor amounts of 43, 42, 33, and 3E kDa proteins. The proportion of smaller proteins increased with the age of the leaf. In some virus preparations only 3 1 kDa capsid protein was detected. In vitro proteolysis of virions with 45 kDa protein produced virions with 31 kDa protein. Virions with 31 kDa capsid protein sedimented slightly more slowly than those with 45 kDa protein. Antiserum to intact virions, containing predominantly 45 kDa protein, reacted with both 45 and 3 1 kDa proteins on Western blots. Thirteen isolates of WSMV could be classified into two groups with major capsid proteins of 47 and 45 kDa, respectively, as well as accompanying minor proteins. Most natural isolates, including Sidney 81, contained 45 kDa protein, but the type strain contained 47 kDa protein.