The present study aimed at evaluation of the immuomodulatory effect of sulphadimidine by investigating its ability to affect constitutive expression of MHC-II molecules on B-cells, which are one determinant of antigen presentation and the vigor of immunity. Three groups of broiler chicken were used, the first two groups received 1 mg/kg and 2 mg/kg sulfadimidine, respectively, in drinking water; while the third group was left as control. Bursal samples were taken after 7 and 14 days of treatment and checked for MHC-II gene expression by RT-PCR technique. The obtained results demonstrate that oral administration of sulphadimidine inhibited the expression of MHC-II mRNA in bursal of Fabricius of broiler chicken in a dose and age dependent manner. This finding is of clinical importance as many farmers routinely add sulpha drugs to drinking water as a prophylactic measure against some infections; yet the data presented in this study doesn't recommend this managing behavior.