Infectious bursal disease continues to pose an important threat to the commercial poultry industry, especially after the emergence of virulent infectious bursal disease virus (IBDV). L-arginine, a ubiquitous, basic amino acid is used as an immunostimulant in variety of human and animal studies with great success in immunosuppressed hosts. In the present study, we evaluated the immunomodulatory effects of L-arginine on humoral and cellular immune response in chickens immunized with live intermediate plus (IP) strain of IBDV vaccine. Chickens vaccinated with IP strain and supplemented with 2% L-arginine showed 100% protection after challenge with virulent IBDV compared to 80% protection induced by IP strain alone. IP strain vaccine elicited good antibody titres in both the groups, IP + L-arginine and IP alone, however, the antibody titres in IP + L-arginine group were significantly higher (P < 0.05) than IP vaccinated chickens. Mitogenic response of peripheral blood lymphocytes to specific IBDV antigen stimulation was significantly higher in IP + L-arginine group (P < 0.05). Peripheral blood lymphocytes were able to rapidly clear the suppressive effects of IP strain and virulent challenge virus in IP + L-arginine group. These results strongly suggest that l-arginine plays a vital role in modulation of protective immune response against IBDV.