Abstract. An ELISA for detection of serum IgM antibodies to the galactose-inhibitable adherence lectin of Entamoeba histolytica revealed that 2.8% of uninfected controls, 0.0% of controls infected with other parasites, 13.4% of asymptomatic amebic infections, 55% of colitis patients, and 77% of amebic liver abscess patients from Cairo, Egypt and Durban, South Africa had serum anti-lectin IgM antibodies. Of acute amebic colitis patients with symptoms for less than one week, only 6% possessed serum IgG anti-lectin antibodies, yet 45% had serum IgM antibodies to the amebic lectin. This compares with 65% of sera in acute colitis patients positive for lectin antigen as determined by ELISA with anti-lectin monoclonal antibodies. In conclusion, an ELISA for serum anti-lectin IgM antibodies appears to have greater clinical utility in the setting of acute amebic colitis than an ELISA for anti-lectin IgG antibodies, but is no more sensitive than an ELISA for detection of lectin antigen in sera.In some protozoal infections, the measurement of IgM antibodies is helpful in identification of current and postinfection states, such as giardiasis 1 and toxoplasmosis. 2 Detection of anti-Entamoeba histolytica IgM antibodies may constitute an important strategy for early diagnosis. Regression of the serum anti-amebic IgM antibody response occurs within three months in 50% of cases and reappearance or a later increase in anti-amebic IgM antibody titer represents an important prognostic factor. 3 The relatively short duration of the anti-amebic IgM antibody response has also been confirmed by Jackson and others 4 compared with the persistence of anti-amebic IgG serum antibodies for years after cure of invasive amebiasis. 5 Serum IgM antibodies to amebic plasma membrane antigens were detected previously with high specificity and sensitivity by an ELISA procedure. 6 Reactivity of anti-amebic IgM antibodies to small (5-15 kD) cytoplasmic membrane antigens has been demonstrated. 6 The use of well-characterized native amebic surface antigens may improve the diagnostic utility of serologic tests. The galactose-inhibitable lectin of E. histolytica mediates adherence of trophozoites to colonic mucins, epithelial cells, and host inflammatory cells. 7-9 Galactose-inhibitable lectin binding is absolutely required for amebic lysis of mammalian cells. 8 We used an ELISA with purified native amebic lectin, 10,11 which contains a highly conserved 170-kD antigen subunit that is recognized by serum IgG antibodies from patients worldwide with invasive amebiasis. 10,12-14 Lectin antigenemia has been demonstrated in patients with invasive amebiasis. 15,16 Amebic lectin has been detected in feces from patients with E. histolytica or E. dispar intestinal infections. 15,17 Previous studies have demonstrated that subjects with other parasitic infections do not generate antibodies to the amebic lectin and that anti-lectin monoclonal antibodies are specific for E. histolytica and E. dispar lectin epitopes. 11,13,15,16 In our study, we determined whether dete...

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