In the last three years, US EPA Methods 1622 and 1623 have demonstrated thatCryptosporidium oocysts and/or Giardia cysts can be recovered reproducibly from small volumes (10 L) of source water. The specific objectives of this study were to characterize oocyst recoveries from each component of the overall procedure for Cryptosporidium oocysts derived from high volume samples of finished water. Presently there are three commercially available anti-Cryptosporidium immunomagnetic separation (IMS) kits manufactured by Dynal A.S. (Dynabeads anti-Cryptosporidium, Norway), ImmuCell (Crypto-Scan, US), and Genera Technologies (Puri-Max, UK). In this paper, the authors discuss the impact of finished water concentrations on IMS performance and discuss the approaches taken to modify existing IMS procedures to handle larger sample volumes than those recommended by their respective manufacturers. Includes 3 references, tables.