- Diagnosis of Cryptosporidium is made by the identification of oocysts in stool specimens. Screening in field studies relies mainly on acid-fast staining followed by microscopic examination. The more sensitive immunofluorescent antibody (IFA) staining method is time-consuming, may involve technical difficulties, and is extremely costly as a screening procedure in field studies. We evaluated the diagnostic utility of a commercially available enzyme immunoassay (EIA), which detects Cryptosporidium-specific antigen, in 204 unprocessed stool specimens obtained from patients less than three years of age from a field study in southern Israel. When compared with the routine screening procedure applied in this field study (screening by acid-fast staining and microscopy after concentration, and confirmation of positive results by IFA), both the sensitivity and specificity were 98%. Of 139 specimens negative by microscopy, 13 (9.3%) were positive by the EIA. Eleven of these were confirmed by inhibition with antibody to Cryptosporidia-specific antigen. The EIA is an important tool for identifying Cryptosporidium in fecal specimens in field studies since it is sensitive, specific, simple to use, and unaffected by the presence of a preservative.