The effect of a modified live Infectious bursal disease (IBO) vaccine containing an intermediate strain of IBO virus on the antibody response of broiler chicks to live Newcastle disease (ND) vaccine was evaluated. Seventy chicks from the same breeder flock were randomly divided into four groups. Group A was simultaneously vaccinated against IBO and ND at day-old. Group B was given IBO and ND vaccines at one and seven days old, respectively. Group C was immunized against ND and IBO at seven and ten days of age, respectively. Group D received only the ND vaccine at seven days old, and served as control. All birds were revaccinated for ND at 21 days of age. The bursa of Fabricius was collected from three chicks in groups A, Band C, five days post-IBO vaccination and from 3, 7, 14 and 21 days old chicks in group 0 and examined for gross and histologic changes. Microscopically, lymphocyte degeneration and depletion, cyst formation, interfollicular edema and hemorrhages were found to be more extensive in the bursa of birds vaccinated against IBO at day-old than in those vaccinated at ten days of age. In contrast, bursa of Fabricius taken from day-old chicks prior to IBO vaccination and those from non-IBO vaccinated chicks showed dense bursal follicles populated with normal lymphocytes. Low to moderate levels of maternal antibodies to ND and IBO viruses were detected in sera collected prior to ND and IBO vaccination by HI and ELISA test, respectively. Primary and secondary antibody responses to ND vaccine were consistently and significantly lower in IBO-vaccinated birds than in the control group. Suppression of humoral immunity was more dramatic in birds given the IBO vaccine at day-old than in birds vaccinated at 10 days of age. Results of the study showed the potential of a modified live intermediate IBO vaccine to cause serious damage to the bursa of Fabricius with consequent depression of antibody response to live ND vaccine when given particularly to day-old broiler chicks with low to moderate levels of maternal antibody to IBO virus.