2. The holes in normal and PNH red cells appear to be identical, under the same conditions. The membrane defects produced by lysis of PNH cells with acidified normal serum (the Ham's test) are identical to those produced by complement lysis with specific antibody, indicating that complement is undoubtedly the cause of such lysis.

3. Evidence is presented that when human complement acts on human red cells sensitized with anti-I antibody, each complete activation of complement leads to the production of a cluster of holes. This contrasts to the action of guinea pig complement, on sheep cells, each activation of which leads to a single hole.

4. The maximum number of anti-I antibody molecules which can attach to a human red cell (i.e. the minimum number of antigen sites) is about 500,000 for both normal and PNH cells.

5. The number of holes produced during lysis of the PNH cell is the same as that of the normal cell. When all cells are lysed by am excess of C', a mean of about 90,000 holes are present on each membrane. When complement is limited, a larger proportion of PNH cells are lysed due to their peculiar sensitivity to C' but the number of holes on each lysed cell is the same as for normal cells lysed by the same concentration of C'.