Indo-Gen mediated surface labelling with125I demonstrated differences in surface oriented antigens between virulent and virulent promastigote ofLeishniania donovani, In case of virulent strains, surface polypeptides with molecular masses of 63, 53, 42 and 38 kDa were found to be labelled with125I whereas in the case of aviralent stains 68, 55, 50, 46, 42 and 33 Da, components were iodinated. Further studies by immunoblot assay using different subcellular fractions of virulent and avirulent parasites demonstrated that antibody raised against gp63 cross-reacted with the 63 and 60 kDa antigen of the virulent and avirulentLeishmania donovani strains of Indian origin respectively. It indicates that these two polypeptides are antigenically similar. When virulent and avirulent cells were grown in the presence of varying concentration of tunicarnycin and immunoblot with anti gp63, it was observed that with increasing concentration of tunicamycin the 63 kDa polypeptide of the virulent cells shifted to approximately 58–57 kDa and the 60 kDa polypoptide of the aviruleni cells shifted to 57 kDa. This suggests that glycosylation may play an important role in antigenic variation between virulent and avirulent parasites.