The objective of this research was to evaluate the serum concentrations of mannan-binding lectin (MBL) at different ages in Ross broilers fed increasing amounts of a yeast-fermented product (YFP) and inoculated with a vaccine against Newcastle disease virus (NDV). Eighty mixed Ross B308 broilers were individually housed. Groups of 20 broilers were randomly assigned to 4 dietary levels of YFP (0, 400, 800, and 1,600 ppm). Serum concentrations of MBL were determined on d 17, 21, 28, and 35. At 21 d, birds were vaccinated against NDV and the titers of antibodies (Ab) against NDV were determined at 35 d of age. There was not changes in the concentrations of MBL due to the increasing levels of dietary YFP, but showed a steady decrease across the sampling d. The NDV-specific Ab titers showed the lowest titers at the highest dietary YFP. Broilers with low (LMBL) and high (HMBL) concentrations of MBL were observed, which exhibited different patterns on the Ab titers against NDV in respect to the increasing levels of dietary YFP. This was also observed when the gender effect was introduced in the analysis of data. The results suggest that in Ross broilers raised in commercial conditions the presence of low Ab titers after a vaccination may be partially caused by factors such as the frequency of LMBL and HMBL chickens, the proportion of females and males in the flock and the additions of dietary growth promoters like YFP.