Live attenuated influenza vaccine (LAIV) consists of reassortant viruses with hemagglutinin (HA) and neuraminidase (NA) gene segments inherited from the circulating wild-type (WT) parental viruses and six internal protein-encoding gene segments from cold-adapted attenuated master donor viruses (genome composition 6: 2). In this study, we describe the difficulties in development of LAIV strains depending on the phenotypic peculiarities of the WT viruses used for reassortment. Genomic-composition analysis of 849 reassortants revealed that over 80% of reassortants based on the inhibitor-resistant WT viruses inherited WT NA as compared to 26% of reassortants based on the inhibitor-sensitive WT viruses. In addition, the highest percentage of vaccine genotype reassortants was achieved when WT parental viruses were resistant to nonspecific serum inhibitors. We show that NA may play a role in the influenza virus' sensitivity to a nonspecific serum inhibitors. Replacing the NA of the inhibitor-sensitive WT virus with the NA of the inhibitor-resistant master donor virus significantly decreased the sensitivity of the resulting reassortant virus to nonspecific inhibitors.