The capacity of polystyrene microspheres with immobilized antibodies against type 1 pili of E. coli was measured. Using pure IgG-type monoclonal antibodies obtained by affinity purification, it was found that, at high concentrations of antibody in solution, 2.6 +/- 0.4 mg of antibody immobilized/m(2) of surface area. The binding of piliating E. coli cells on incubation with antibody-coated microspheres at various microsphere-to-cell number ratios ranging from 1-3750 was studied. It was found that a maximum of 290 +/- 50 cells were bound/microsphere, equivalent to a binding capacity of 2.9 +/- 0.7 x 10(10) cells/m(2) of surface area.