The uptake in monocytes of monodispersed latex microbeads precoated with whole bacillus Calmette-Guérin (BCG) cells, Mycobacterium tuberculosis sonicate or culture fluid, or antigen (Ag) from the culture fluid was examined by microscopy. There was a significantly higher cell association of beads coated with whole BCG cells, the secreted 85C component of the Ag 85 complex or M. tuberculosis sonicate than of phosphate-buffered saline (PBS)-treated control beads. Antibodies (Ab) to Ag 85 inhibited the uptake of BCG- and Ag 85C-treated beads. A monoclonal antibody (mAb) to complement receptor type 3 (CR3), but not mAb to CR1, inhibited the uptake of Ag 85C-coated beads, indicating that the mycobacterial Ag-dependent uptake of particles was mediated via CR3 on the monocytes. This points to the existence of a ligand on Ag 85C which may promote monocyte uptake of M. bovis, BCG and M. tuberculosis.