PNH cells are known to lyse in normal human serum (NHS) at acid pH (Ham's test) but not (or very little) in serum at physiologic pH. When PNH cells are treated with neuraminidase (PNH-NA) their lysis in acid serum is increased (Yachin, S. et al., Blood 17  83); in addition they lyse (10 to 58%) in ABO compatible NHS at physiologic pH. There are three possible explanations for the lysis of PNH-NA in NHS to consider: a) An antibody to PNH-NA is present in NHS; b) PNH-NA are inherently more susceptible to complement lysis than PNH; or c) PNH-NA actively participate in the lytic process by serving as an activator of the alternative complement pathway. Against a), absorption of the serum at 0°C three times with PNH-NA did not appreciably remove its lytic activity. And against b), PNH and PNH-NA lysed equally well in ABO incompatible serum (a source of antibody and complement), and they lysed equally well when used as indicator cells in a CoVF-reactive lysis assay.