Abstract
The immobilizing effect of normal guinea pig serum and some serum fractions on P. aurelia is correlated with their haemolytic effect on sensitized erythrocytes, indicating that the activity is due to complement. Complement fixation tests were used to compare P. aurelia and T. pyriformis antigens. Complement was found to be fixed by the external antigens of T. pyriformis but not by those of P. aurelia. Breis formed by macerating both species of ciliate fixed complement and cross-reactions between the two breis and the non-homologous antisera were shown to occur. No difference, as shown by complement fixation tests, was found between the internal antigens of two varieties of P. aurelia. The effect of complement on the immune reactions involving P. aurelia and T. pyriformis and their homologous antisera was examined and no lytic reaction at high dilutions of the antisera was found.
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