Bordetella pertussis produces a number of virulence determinants believed to contribute to its survival in the host as well as to the pathogenesis of disease. One of these factors, adenylate cyclase toxin (ACT), has been implicated to penetrate human neutrophils and macrophages and abrogate their function by virtue of unregulated production of intracellular cAMP. In order to adequately study the nature of ACT and its role in pathogenesis, it is necessary to isolate the toxin from other virulence factors produced by the organism. Attempts by other investigators to purify ACT and maintain both its invasive and catalytic properties have not been successful. B. pertussis produces a cell associated ACT during mid-log phase of growth in Stainer-Scholte medium. Purification of ACT with both activities from urea extracted whole cells has been achieved by hydroxylapatite and calmodulin-sepharose chromatography. ACT is a single protein of 220 kd molecular weight with an isoelectric point of 7.0. The protein probably contains regions which are strongly hydrophobic. ACT has a specific activity of nearly 17,000 μM cAMP formed/min. An 850 ng sample of ACT induced over 1,400 pmoles cAMP/10⁶ S49 mouse lymphoma cells while 660 ng of ACT inhibited human neutrophil chemiluminescence by 65%.