This paper describes a portion of the data that will ultimately be used in the development of an anitbody evaluation protocol. Lot to lot variability, cross reactivity, and avidity of four commercially available antibodies were evaluated using flow cytometer-derived fluorescence measurements. The ability of each antibody to bind Cryptosporidium parvum and Giardia lamblia following laboratory procedures including calcium carbonate flocculation, ICR and Method 1622 processing, and fixation was also quantified. In these studies calcium carbonate flocculation was shown to decrease fluorescence intensity of C. parvum oocysts regardless of the antibody tested; however, fluorescent staining of G. lamblia cysts processed by this method was minimally affected. Cysts and oocysts processed by the ICR method or Method 1622 showed no or small staining differences although some of the small differences were statistically significant. Lot to lot variablility was greatest with the Techlab and WaterBorne antibodies while the HydroFluor and MeriFluor antibodies stained cysts and oocysts more consistently. Cross reactivity to non-parvum species was greatest with the WaterBorne and the Techlab antibodies; the HydroFluor and Techlab antibodies each cross reacted with G. muris to some degree. Formalin fixation decreased fluorescence intensity of parasites stained with each antibody tested. The final stages of this project will involve a performance evaluation that will determine the role of antibody in recovery of parasites seeded into environmental samples varying in turbidity and composition. The results of this performance section, together with the data described above will be pooled to develop an antibody evaluation protocol and provide guidance to antibody manufacturers.