A commercially available enzyme-linked immunosorbent assay (ELISA) for the detection of Giardia lamblia in stool specimens was evaluated on 342 specimens submitted to the Mayo Clinic Parasitology Laboratory for routine examination. The stools were either fresh or fresh/frozen at -65 degrees C (139 specimens) or were preserved in formalin (203 specimens). ELISA results were compared with those obtained by conventional microscopic examination: 143 stools were positive by both methods and 186 were negative. Sixty-six of the negative specimens contained 96 parasites belonging to 16 species other than Giardia, indicating a low rate of cross-reactivity. There were eight "false positive" ELISA results, which included specimens from one patient who had previously had a "true positive" and another patient with multiple family members infected with Giardia. Five stools that were "falsely negative" by ELISA contained only rare G. lamblia. The ELISA sensitivity was 97%, the specificity was 96%, and the positive predictive value was 95%. Results were evenly distributed between frozen and formalinized stools. The LMD/Seradyn ELISA appears to be a simple, rapid, and accurate method for the detection of G. lamblia in unprocessed stool samples.