Specific receptors may be involved in the process of attachment of Leishmania donovani promastigotes to macrophage surfaces and their subsequent internalization. Two virulent strains of Indian L. donovani (AG83 and GE-I) were found to enter into macrophages much faster than the avirulent ones (UR6). These virulent promastigotes express surface glycoprotein (gp63) and lipophosphoglycan (LPG) to a greater extent than avirulent strains. We examined their interaction with macrophages as a function of time by preblocking the macrophage receptors with the exogenous addition of gp33 or LPG. In experiments where gp63 was used as the blocking agent, the entry of one virulent strain (GE-I) was affected. In other experiments where LPG was used, the entry of another virulent strain (AG83) was affected. Entry of the avirulent strain (UR6) was unaffected by either of these treatments. Exposed LPG or gp63 on the surface of promastigotes thus appear to expedite their recognition and entry into the host cell. To assess the role of gp63 further in the entry of Leishmania into the macrophages, an avirulent UR6 strain was transfected with the gp63 gene cloned from L. amazonensis. The transfected UR6 as expected expressed more GP63 at a faster rate and entered into the macrophages like the virulent strain when compared to the nontransfected UR6 or UR6 transfected with vector alone. Thus, the expression of the gp63 gene is involved in the recognition and intracellular entry of visceral Leishmania into the macrophages in addition to the cutaneous species demonstrated previously.