Six monoclonal antibodies (MAbs) to pertussis toxin (PT) have been generated and characterized. Five of the MAbs interact with determinants on the catalytic (S1) subunit of PT, and one is specific for subunit S4. The MAbs were divided into three groups based upon their ability to neutralize the effects of PT in a Chinese Hamster Ovary (CHO) cell assay. Three MAbs had high neutralization titers, one had weak neutralizing capability, and two were non-neutralizing. The three neutralizing MAbs cross-reacted with each other, suggesting that all the neutralizing MAbs reacted with the same or adjacent epitopes. Additional experiments utilizing tryptic digests of the S1 subunit or truncated S1 proteins produced by recombinant expression in E. coli cells suggested that the neutralizing epitope recognized by the three MAbs was a conformational epitope. This conclusion was supported by additional experiments which demonstrated that the neutralizing MAbs did not bind to any of 45 synthetic peptides which covered the entire S1 subunit structure. These MAbs have proved to be very useful in a number of research projects aimed at identification of areas in the PT molecule which are of importance with respect to the design of acellular pertussis vaccines. One of the MAbs is being evaluated for use as an International standard for PT ELISA assays. Two additional MAbs were produced against a synthetic peptide corresponding to a PT sequence which has considerable sequence homology to a region in the cholera toxin sequence. These antibodies have been characterized and it has been shown that they are capable of recognizing the appropriate regions in the two toxins, however neither is capable of neutralizing toxin activity. These antibodies are currently being used to probe the function of this homology region with respect to the mechanism-of-action of these two toxins. All of these MAbs have been provided to numerous research laboratories outside the FDA and at least two manuscripts have been published by these laboratories utilizing these MAbs.