The serums of ninety patients observed at either The Mayo Clinic or the Vanderbilt University Hospital, all harboring Endamoeba histolytica, were tested for complement fixation with a variety of amebic antigens. The serums were examined independently in the laboratories of two institutions. There was general agreement in the findings of the two laboratories, but owing to the use of multiple antigens one laboratory was able to obtain more positive reactions than the other. Considering only tests which resulted in strong fixations found in either laboratory, twenty-nine of the ninety cases, or 32 per cent, gave positive complement fixation reactions. The average sensitivity of all tests now reported in the literature is 68 per cent. The two laboratories failed to check in 16.7 per cent of the tests. Cases of acute amebiasis gave a distinctly higher percentage of positive results than any other group of cases, but there were cases in which tissue was demonstrated to have been invaded by Endamoeba histolytica in which the reactions were negative. A considerable improvement in the preparation of antigen will be necessary before the test can be generally adopted.