Little is known on the interactions between avian influenza virus (AIV) and Newcastle disease virus (NDV) when coinfecting the same poultry host. In a previous study we found that infection of chickens with a mesogenic strain of NDV (mNDV) can reduce highly pathogenic AIV (HPAIV) replication, clinical disease, and mortality. This interaction depended on the titer of the viruses used and the timing of the infections. To further explore the effect of mNDV infectious dose in protecting chickens against HPAIV infection, 2-wk-old birds were inoculated with different doses of mNDV (10⁴, 10⁶, or 10⁷ 50% embryo infective dose [EID₅₀]) 3 days before inoculation with a HPAIV (10⁵ or 10⁶ EID₅₀). Although birds coinfected with the higher mNDV doses (10⁶ or 10⁷) survived for longer than birds inoculated only with HPAIV (10⁵), we did not observe the same protection with the lower dose of mNDV (10⁴) or when given the higher dose of HPAIV (10⁶), indicating that the relation between the titer of the two coinfecting viruses is determinant in the outcome. In a similar experiment, a higher number of 4-wk-old birds survived, and for longer, even when given higher HPAIV doses (10^6.3 and 10^7.3 EID₅₀). In addition, we also examined the duration of protection provided by mNDV (10⁷ EID₅₀) on a HPAIV infection. Five-week-old chickens were inoculated with mNDV followed by inoculation with 10⁶ EID₅₀ of an HPAIV given at 2, 4, 6, or 9 days after the mNDV. HPAIV replication was affected and an increase in survival was found in all coinfected groups when compared to the HPAIV single-inoculated group, but the mortality in coinfected groups was high. In conclusion, previous inoculation with mNDV can affect HPAIV replication in chickens for at least 9 days, but this viral interference is titer dependent.