This work demonstrated that an HCT-8 cell culture-based infectivity coupled with RT-PCR for detecting C. parvum infections is a practical tool that can provide valuable information about the efficacy of disinfectants and the infectivity of oocysts in environmental waters. It is a widely applicable technique that supports the growth of the two primary genotypes of C. parvum and has been shown to be equivalent to the gold standard of mouse infectivity. The results with five isolates demonstrated that infectivity in cell culture is a good indicator of the ability of oocysts to cause infection in animals. However, the choice of cell line and the method used to detect infection is important in determining equivalency. This would also be the case with different mouse models since some strains demonstrate different sensitivities to infection. Ultraviolet disinfection (UV) is an extremely promising disinfection technology which may greatly reduce the risk to public health posed by waterborne cryptosporidiosis. The disinfection data presented in this report demonstrates that the HCT-8 cell culture/RT-PCR assay was also equivalent to a mouse assay for measuring UV inactivation of C. parvum oocysts. Therefore, in-vitro cell culture-based assays should be regarded as practical alternatives to the use of animals for measuring the infectivity of C. parvum and assessing the efficacy of disinfectants. However, cell culture-based disinfection assays must be designed with sufficient replication and statistical robustness to allow confidence in the results. The natural variability within infectivity assays, the identification of at least two primary genotypes of C. parvum with different host ranges, and the variation in dose response curves for different isolates all suggest that a cautious approach should be adopted when interpreting cell culture or animal-based inactivation data. Future cell culture-based inactivation studies should investigate the correlation between cell culture and animal assays for a broader range of disinfectants (e.g., ozone and chlorine dioxide). In addition, a larger number of cell monolayers for each data point needs to be part of the experimental design and a wider diversity of isolates belonging to both of the primary genotypes needs to be studied. Also, there should be efforts to standardize cell culture-based infectivity assays (e.g., using oocysts of the same age and produced under the same conditions) so that comparisons can be made between studies. In addition to standardization, future work should include an evaluation of infectivity in cell culture compared to infection in human volunteers and a thorough comparison of the different methods available for detecting and quantitating infections in cell culture. Includes 28 references, figures.