Hemolysin of Pseudomonas aeruginosa has a cytopathic action on blood and tissue culture cells. Lysis and disintegration of the architecture of the cell involving membrane and cytoplasm were demonstrated by morphological changes. The hemolytic activity of hemolysin is inhibited by normal sera and by albumin; inhibition is also observed in the action of hemolysin on K.B. cells, but this inhibition is not complete. The hemolytic and cytopathic actions are explained by assuming that they alter the molecular architecture of the membranes. The variability may relate to the differing availability of reactive sites on the cells, or indicate that the two activities are associated with two different enzymes.