Optimum conditions are described for the complement fixation tests, including the method of primary incubation in a refrigerator, use of serial four-fold dilutions for complement titration and serial double dilutions for haemolysin titration. An optimum haemolytic system, most sensitive to the complement and 50% unit of complement is to be found at the same time. More than 100 sera were tested simultaneously using other reactions for toxoplasmosis (microprecipitation in gel, indirect fluorescence reaction and latex test) in order to verify the appropriate dose of complement for complement fixation reaction. It was also investigated to what extent small deviations in erythrocyte concentration in the haemolytic system may influence the results of serum titration. The appropriate dose of complement was found to be five 50% units. It is recommended to correct colorimetrically the concentration of erythrocytes in the haemolytic system. The questions of an appropriate serum dilution in the titration are discussed and a rapid and exact method is chosen. A simple method of reading 50% haemolysis in complement fixation microreaction is described.