Immunologic methods for detection of colorectal neoplasia based on examination of stool or colonic effluent are being developed. Most current oral lavage preparations contain polyethylene glycol (PEG), and if PEG adversely interferes with immunologic testing these tests may become less useful. We describe a decrease in sensitivity of ELISA for tumor-associated antigens (TAA) when effluent samples are diluted in PEG-electrolyte lavage solution, equivalent to a commonly used oral lavage solution based on PEG. Radioisotope-labeled antigen binding to plastic plates was decreased by dilution in the PEG lavage solution. Antigen binding, present in colonic effluent collected by the laxative purge method, was absent in effluent collected by PEG oral lavage from the same patient. We conclude that PEG and PEG-containing lavage solutions interfere with ELISA detection of TAA in colonic effluents. We speculate that the in vitro, and possibly the in vivo, effect occurs at the level of antigen binding to the plate either by a steric effect or alteration of charge by the nonpolar properties of PEG.