We examined the role of the macrophage receptor for C3bi, the CR3, in the phagocytosis of Leishmania major promastigotes and report that M1/70, a monoclonal antibody to the CR3, inhibited the binding of leishmania to macrophages both when the assays were performed in the presence of normal serum and in its absence. In serum, leishmania activate complement and fix C3. Fixation and subsequent cleavage to C3bi occurs rapidly, and by as early as 5 min both forms of the molecule can be identified on the parasites' surface. Complement fixation results in an enhanced phagocytosis of leishmania promastigotes by mouse macrophages. In the case of L. major, 63% of this serum-enhanced binding is inhibitable by M1/70. Binding assays were also performed in the absence of serum with the use of thoroughly washed promastigotes. The addition of M1/70 inhibited binding under these conditions by 54%. Two other rat monoclonal antibodies directed against different antigens on the macrophage plasma membrane did not inhibit binding. M1/70 did not inhibit the binding of promastigotes to rat bone marrow cells, nor did it inhibit IgG-SRBC binding to mouse peritoneal macrophages. These data indicate that the inhibition observed in the presence of M1/70 was specific for the CR3 and that the macrophage receptor for C3bi plays a major role in the phagocytosis of Leishmania major promastigotes, even in the absence of serum.