This report presents the results of the U.S. Environmental Protection Agency's (EPA's) interlaboratory validation study (the 'Study') of Method 1623: Cryptosporidium and Giardia by Filtration/IMS/FA (the 'Method'). The purpose of this Study was to determine the precision and recovery for Cryptosporidium and Giardia in reagent water and raw surface matrices in multiple laboratories using the Method. One referee laboratory and 11 participating laboratories were involved in the Study. The Study was conducted in February 1999. The referee laboratory used a flow cytometer to sort a known number of unstained Cryptosporidium oocysts and unstained Giardia cysts into spiking suspension containers. Six single-blind spiking suspensions and one double-blind reagent water blank were distributed to each participating laboratory. Each spiking suspension contained approximately 129 oocysts and 129 cysts. Each laboratory analyzed four spiked reagent water samples, one reagent water blank, two spiked raw surface water samples, and one unspiked raw surface water sample according to the December 1998 version of the Method, as amended by technical clarifications. These clarifications have been incorporated in the final version of the Method.