A fowl plague virus (FPV) temperature-sensitive mutant ts 5 having mutation lesions in the gene coding for the neuraminidase has been obtained. The mutant induced synthesis of cRNA, vRNA and proteins in cells under non-permissive conditions, but formation of virions including non-infectious ones was defective. The neuraminidase and haemagglutinin synthesized under non-permissive conditions possessed functional activity and could migrate from the rough endoplasmic reticulum into plasma membranes; however, cleavage of the haemagglutinin was reduced. In ts 5-infected cells under non-permissive conditions the synthesis of segments 5 and 8 of cRNA and vRNA was predominant both early and late in the reproduction cycle, and the synthesis of P1, P2, P3, HA and M proteins was reduced after approximately 3 hours. The data obtained suggest that involvement of the neuraminidase in the formation of infectious virions may have no direct association with the enzymatic activity of this protein, and that the mutation in the neuraminidase may affect regulation of replication and transcription processes.