Two populations of Leishmania donovani chagasi promastigotes resistant to the lethal effects of tunicamycin (TM), an inhibitor of N-linked glycosylation, were raised. These parasites exhibited altered patterns of glycosylation when compared to wild-type controls. In particular the major surface glycoprotein gp63 was present in membranes of one population of TM-resistant promastigotes (population 1) primarily in a deglycosylated form, which migrated at a lower Mr than wild-type gp63. The deglycosylated protein was proteolytically inactive on substrate-containing gels, in contrast to glycosylated gp63. Assays of promastigote attachment to human macrophages revealed that, despite a proposed role for gp63 protease activity in binding to macrophage receptors, population 1 TM-resistant promastigotes appeared to attach to the CR3 but not to the mannose-fucose receptor. Control promastigotes bound to both receptors. In contrast, a second population of TM-resistant promastigotes (population 2) did not produce gp63 that could be detected on immunoblots, either in a glycosylated or deglycosylated form. The latter TM-resistant promastigotes bound to neither CR3 nor the mannose-fucose receptor, suggesting that expression of gp63 might be important for promastigotes to bind to CR3. LPG was present in immunoblots of both TM-resistant and control populations suggesting this molecule might not account for the differences in attachment. Surprisingly both TM-resistant promastigote populations contained gp63 mRNA by Northern analysis in apparently equal amounts. We conclude that N-glycosylation is probably necessary for the protease function of gp63, but that neither N-glycosylation nor protease activity of gp63 is necessary for L. donovani chagasi promastigotes to bind to CR3. Furthermore the expression of gp63 protein by TM-resistant promastigotes is dependent upon postranscriptional events.