1. At a temperature of 1 degrees C., C'2, C'4, and C'1 combine with sensitized sheep erythrocytes, while C'3 does not combine. 2. C'1, although combining with sensitized cells in the absence of C'4, is hemolytically inert unless C'4 combines previous to, or simultaneously with it. 3. C'4 does not combine in the absence of C'1. 4. Although C'3 is not fixed by antibody-sheep cell aggregates, it is essential for hemolysis, operating on the sensitized red cell after the fixation of C'4, C'2, and C'1 and behaving as if it were a catalyst.