The replication and polypeptide synthesis of orbivirus isolate Mill Door/79, a member of the Kemerovo serogroup, were studied. In Vero cells cell-associated virus exceeded cell-free virus by about 2 log10 PFU/ml. Attempts to purify the virus resulted in the demonstration of five polypeptides. Thirteen virus-induced polypeptides and 10 segments of double-stranded RNA were identified in infected cells. Partial proteolysis demonstrated homology between some polypeptides.