Antigen opsonization by the C3b fragment of complement is a significant event in the modulation of cellmediated immune response, but its mechanism is still largely unknown. The structural characteristics of C3b allow it to act as a bifunctional ligand between antigen and cells via their membrane C3b receptors. It was thus of interest to study the influence of the covalent link between C3b and antigen on the fixation and internalization of this antigen by antigen-presenting cells. Tetanus toxin (TT) was used as antigen, either free or covalently linked to C3b (TT-C3b). The antigen-presenting cells were TT-specific (4.2) or nonspecific (BL15) Epstein–Barr virus (EBV)-transformed B cells. C3b was found to play an important role in antigen fixation and internalization by both antigen-specific and antigen non-specific cells. Covalent binding of C3b on TT (1) permitted fixation and internalization of this antigen by non-specific cells via their complement receptors; (2) enhanced antigen fixation and resulted in cross-linking between membrane immunoglobulins and complement receptors on antigen-specific cells. The consequences of covalent C3b binding to TT were analysed using antigen-specific and antigen-non-specific cells. In both cases, a net increase in antigen fixation was observed. At the intracellular level, covalent C3b binding to TT resulted in a large TT incorporation in endosomes of non-specific cells, similar to that observed in antigen-specific cells. Thus, C3b covalently linked to antigen enlarges the array of B-cell types capable of presenting antigen, including non-specific cells.