Cryptosporidium parvum is now considered one of the most important agricultural contaminants of water worldwide. Although costs associated with removal of this pathogen from drinking water are not yet available for most countries, estimates ranging from 25 to 66 million pounds annually are projected for the United Kingdom. In order to detect whether viable oocysts are present within drinking water supplies, the use of in vitro cultivation systems are commonly employed. Significant advancements in the in vitro cultivation of Cryptosporidium parvum have been achieved within the last decade, and today large numbers of asexual and sexual stages can be grown in cell cultures. However, despite the impressive bioamplification that can be achieved early in the life-cycle of the parasite, generation of significant numbers of oocysts in these cell culture systems still remains a long range goal. Multiple factors influence parasite development in vitro, including age of oocysts, strain of oocyst, types of purification procedures employed, inoculation dose, host cell type, and medium supplements. Any one of these factors may make the difference between success or failure of an in vitro system. The following represents a synopsis of several important parameters affecting growth of this parasite in cell culture, and presents data demonstrat ing that even slight variations in protocol may have a pronounced effect on the success or failure of an experiment. Includes 24 references, table, figures.