The objective of this study was to investigate the effect of disinfectant concentration and pH on the inactivation kinetics of Cryptosporidium parvum oocysts with ozone, monochloramine, and ozone/monochloramine at 20 degrees C. Experimental results revealed that the CT (product of disinfectant concentration and contact time) required to achieve a certain level of C. parvum inactivation was unique, thus demonstrating the validity of the CT concept for these single disinfectant and sequential disinfection processes for the range of experimental conditions investigated. Inactivation curves were represented accurately by a delayed Chick-Watson expression consistent with the CT concept. No pH dependence was observed for primary inactivation with ozone in the pH range of 6-10 or primary and secondary inactivation with monochloramine at pH values of 8 and 10. Oocyst resistance to chemical disinfectant attack was found to vary among oocysts lots as well as with oocyst aging within a given lot. The synergy observed for sequential disinfection with ozone/monochloramine suggested that monochloramine might be reacting with some of the same chemical constituents, both vital and nonvital, of the oocyst wall and/or cavity that also react with ozone. If so, partial completion of these reactions by the primary disinfectant would have resulted in the disappearance of the lag phase and the faster rate of inactivation observed for the secondary disinfectant.