Summary: Changes in the degree of sensitization of the erythrocytes or of the hemolysis time are shown to affect the activities of several complement solutions to different extents. These peculiarities are attributed to differences in the C component ratios of the solutions. It is suggested that two functional activities are involved in C interactions, one taking part in fixation and the other involved in a subsequent hemolytic attack on the erythrocyte. The apparent anomalies in C activity are attributed to the interactions of these two agents.