Invasive, disease-associated members of the Mycobacterium avium complex are facultative intracellular pathogens of mammalian macrophages. The mechanism(s) by which M. avium is ingested by mononuclear phagocytes is unknown. We examined the role of membrane receptors on macrophages as well as the role of opsonic components of the serum (complement, fibronectin, C-reactive protein and fibrinogen in the attachment and ingestion of M. avium by human monocyte-derived macrophages. Preincubation of serum with antibodies against C3 and fibronectin, in contrast to preincubation of serum with antibodies against complement-reactive protein and fibrinogen, significantly reduced the binding of M. avium to macrophages in concentration-dependent manner (57 to 93% and 35 to 61% inhibition by anti-C3 and anti-fibronectin antibody, respectively, in a concentration range of 25 to 100 micrograms/ml). We also observed that incubation of macrophages with OKM1 anti-complement receptor type 3 (CR3) antibody in the presence of serum decreased the binding of M. avium to macrophages by 86% +/- 6%, while the same antibody inhibited binding by 63% +/- 7% in the absence of serum. In contrast, incubation of macrophages with anti-LFA-1, anti-p 150.95, anti-CR1, or anti-Mac-1 had no effect on the ability of M. avium to bind to the cell. In addition, incubation of macrophages with alpha-methyl-D-mannoside was also associated with decreased attachment of M. avium to macrophages. These results provide evidence for the role of three macrophage receptors (CR3, fibronectin, and mannosyl-fucosyl receptors) in the uptake of M. avium by human macrophages.