Capsid protein CAp30 and nucleocapsid protein NCp10 of Moloney murine leukemia virus (MoMuLV) are the 2 major proteic components of the virion core and are generated by processing of the gag polyprotein precursor, Pr65gag, by the viral protease. In the virion core, several hundred NCp10 molecules are bound to the genomic RNA dimer forming the nucleocapsid structure. In the course of virus assembly, NC protein, as the mature NCp10 and/or as the gag precursor, appears to direct genomic RNA packaging. In vitro, NCp10 has nucleic acid binding and annealing activities and promotes viral RNA dimerization and the annealing of replication primer tRNAPro to the primer binding site (PBS) which is necessary for the initiation of reverse transcription. To investigate whether maturation of NCp10 is required for virus formation, we substituted charged residues for the hydrophobic amino acids at the capsid-nucleocapsid protein cleavage site in order to prevent maturation of NCp10. Here we report that these mutations abolished maturation of capsid protein CAp30 and NCp10 by the viral protease in vitro. When these mutations were introduced into an infectious MoMuLV molecular clone, Pr65gag precursor was synthesized in transfected cells but virion production was strongly diminished and mutant viruses were not infectious. These results suggest that maturation of NCp10 is required for optimal virion release and production of infectious virus.