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Complement fixing reactions were observed between normal guineapig serum and antigens prepared from flukes of the genus Fasciola. This activity did of any other serum, and at the usual relative concentrations of antigen and serum, the guinea-pig serum was the limiting component. The effect therefore differ from a normal anticomplementary activity at the was termed the autofixation effect. In the presence of a gross excess of the antigen, the autofixation effect was inhibited. The antigenic component concerned was demonstrated in preparations from both Fasciola hepatica and F. gingantica, from adult and immature flukes, and from the metabolic products of F. gigantica. It was heat-stable and precipitated in 50 per cent ethyl alcohol, hut was soluble in water saturated with ether, Normal sent could be used to absorb the component out of the antigen. The autofixation effect summated with the normal anti-complementary activity of the antigens. In the 'Quantative Complement Fixation Analysis' type of complement fixation test, the control set up to allow for the anticomplementary activity of the antigen will also compensate for this effect. This does not apply with other forms of complement fixation test. [ABSTRACT FROM AUTHOR]