Summary and Conclusions In order to evaluate the two methods, a total of 658 intradermal and 161 complement fixation tests was conducted on onchocercosis patients in endemic areas and on control individuals in other areas. A single skin biopsy on 81 individuals with Onchocerca nodules disclosed microfilariae in 40 per cent of the cases, whereas O. volvulus antigen in a dilution of 1:8,000 gave a positive reaction in 78, or 96.2 per cent, of the individuals when the reading of the reaction was based on an antigen wheal greater by 3 mm. or more than the control wheal. In a dilution of 1:8,000, antigens prepared from Dirofilaria immitis, Setaria equina, and Litomosoides carinii gave a lower percentage of positives; the efficacy of these antigens was exceeded by a 1:16,000 dilution of O. volvulus material. The O. volvulus antigen was more specific than the other antigens. In a series of 50 patients (Morganton, N. C.) who had never been exposed to O. volvulus infection but who harbored other helminth parasites, false positive reactions were obtained with a 1:8,000 dilution in the following number of cases: D. immitis 10, S. equina 11, L. carinii 6, and O. volvulus 2. In a Mexico City group of 209 individuals not exposed to O. volvulus but of whom 92 were infected with intestinal helminths, twice as many false positive reactions were obtained with D. immitis and S. equina antigens than with the O. volvulus material. Complement fixation reactions with O. volvulus antigen proved positive in all of 60 individuals with Onchocerca nodules; D. immitis antigen gave positive reactions in 49 of 53 of these cases. In a control group of 43 persons infected with helminth parasites other than O. volvulus, D. immitis material gave positive complement fixation tests in 4 cases, a doubtfully positive reaction in 1 case, and a negative reaction in 39 cases. Intradermal and complement fixation tests on a series of school children at Tamazunchale, Mexico, not supposed to be within the endemic area of onchocercosis, gave a higher percentage of positive reactions than obtained in individuals in any other control group. At the present time no satisfactory explanation can be given for the significant difference in the number of positives obtained in the control individuals at Tamazunchale and Mexico City. From the results in all series, it appears that the O. volvulus antigen is more specific and sensitive than the other three antigens employed in the intradermal tests. The second best antigen appears to be that from D. immitis. The O. volvulus antigen is also superior to the D. immitis antigen when employed in the complement fixation test. Because of the lack of material, judgment on the comparative value of L. carinii antigen in complement fixation tests must await further work. Antigen prepared from S. equina invariably gave anticomplementary reactions. Quantitative complement fixation tests are suggested as a method of measuring the efficacy of drugs in studies on the chemotherapy of onchocercosis.