The invention relates to an in vitro method ofrescuing negative RNA virus from low virulent virus strain(s) or lentogenic- like virus strain(s) belonging to the order Mononegavirales including Bomaviridae, Filoviridae, Mymonaviridae, Nyamiviridae, Paramyxoviridae, Pneumoviridae, and Rhabdoviridae families, preferably belonging to the Paramyxoviridae family, comprising at least the steps of: (i) co-transfecting hast cells with a two-plasmid system comprising a. a pGenome plasmid comprising at least a sequence encoding a virus genome from low virulent virus strain(s) or lentogenic-like virus strain(s) and b. a pNPL helper plasmid comprising at least the sequences encoding the structural viral proteins nucleocapsid protein (N), phosphoprotein (P) and large protein (L), (ii) culturing hast cells under conditions for replication and transcription of the virus, and (iii) recovering the rescued negative strand RNA viroses.