We have succeeded in engineering changes into the genome of influenza B virus. First, model RNAs containing the chloramphenicol acetyltransferase gene flanked by the noncoding sequences of the HA or NS genes of influenza B virus were transfected into cells which were previously infected with an influenza B helper virus. Like those of the influenza A viruses, the termini of influenza B virus genes contain cis-acting signals which are sufficient to direct replication, expression, and packaging of the RNA. Next, a full- length copy of the HA gene from influenza B/Maryland/59 virus was cloned. Following transfection of this RNA, we rescued transfectant influenza B viruses which contain a point mutation introduced into the original cDNA. A series of mutants which bear deletions or changes in the 5' noncoding region of the influenza B/Maryland/59 virus HA gene were constructed. We were able to rescue viruses which contained deletions of 10 or 33 nucleotides at the 5' noncoding region of the HA gene. The viability of these viruses implies that this region of the genome is flexible in sequence and length.