Six monoclonal antibodies (MAbs) to pertussis toxin (PT) have been generated and characterized. Five of these MAbs (3CX4, 3C4D, 6D11C, 6FX1, and X2X5) interact with determinants on the catalytic subunit (S1) of PT, and one (6DX3) is specific for subunit S4. The MAbs are divided into three groups based upon their ability to neutralize the effects of PT in a Chinese hamster ovary (CHO) cell assay. Three of the MAbs (3CX4, 3C4D and 6D11C) had high neutralization titers, one MAb (6FX1) displayed weak neutralizing activity, and two MAbs (X2X5 and 6DX3) had no neutralizing ability. The combination of one of the high titer MAbs (3CX4) with the low titer MAb (6FX1) resulted in a synergistic enhancement of neutralizing capability. F(ab')2 fragments prepared from MAb's 3CX4 and X2X5 displayed activities in the CHO-cell assay which were identical to the native MAb's. The ability of the MAbs to neutralize PT in the CHO-cell toxin neutralization assay correlated with their ability to inhibit the in vitro ADP-ribosylation of PT. A competition ELISA method demonstrated that this panel of MAbs recognizes at least four separate epitopes on the PT molecule. Biotin-conjugated MAbs were shown to be useful reagents to probe the interaction of pertussis toxin with fetuin.