Guanylyl- and methyltransferases, isolated from purified vaccinia virus, were used to specifically label the 5' ends of the genome RNAs of influenza A and B viruses. All 8 segments were labeled with [α-32P]guanosine 5'-triphosphate or S-adenosyl[methyl-3H]methionine to form 'cap' structures of the type m7G(5')pppN(m)-, of which unmethylated (p)ppN- represents the original 5' end. Further analyses indicated that m7G(5')pppA(m), m7G(5')pppA(m)pGp, and m7G(5')pppA(m)pGpUp were released from total and individual labeled RNA segments by digestion with nuclease P1, RNase T1, and RNase A, respectively. Consequently, the 5'-terminal sequences of most or all individual genome RNAs of influenza A and B viruses were deduced to be (p)ppApGpUp. The presence of identical sequences at the ends of RNA segments of both types of influenza viruses indicates that they have been specifically conserved during evolution.