Adenovirus production is currently operated at low cell density because infection at high cell densities still results in reduced cell-specific productivity. To better understand nutrient limitation and inhibitory metabolites causing the reduction of specific yields at high cell densities, adenovirus production in HEK 293 cultures using NSFM 13 and CD 293 media were evaluated. For cultures using NSFM 13 medium, the cell-specific productivity decreased from 3,400 to 150 vp/cell (or 96% reduction) when the cell density at infection was increased from l to 3 x 10(6) cells/mL. In comparison, only 50% of reduction in the cell-specific productivity was observed under the same conditions for cultures using CD 293 medium. The effect of medium osmolality was found critical on viral production. Media were adjusted to an optimal osmolality of 290 mOsm/kg to facilitate comparison. Amino acids were not critical limiting factors. Potential limiting nutrients including vitamins, energy metabolites, bases and nucleotides, or inhibitory metabolites (lactate and ammonia) were supplemented to infected cultures to further investigate their effect on the adenovirus production. Accumulation of lactate and ammonia in a culture infected at 3 x 10(6) cells/mL contributed to about 20% reduction of the adenovirus production yield, whereas nutrient limitation appeared primarily responsible for the decline in the viral production when NSFM 13 medium was used. Overall, the results indicate that multiple factors contribute to limiting the specific production yield at cell densities beyond 1 x 10(6) cells/mL and underline the need to further investigate and develop media for better adenoviral vector productions. (C) 2009 American Institute of Chemical Engineers Biotechnol. Prog., 26: 200-207, 2010