Luk-I produced by Staphylococcus intermedius was found to be a new member of the staphylococcal bi-component pore-forming toxin family, in which staphylococcal leukocidin, Panton-Valentine leukocidin, and gamma-hemolysin are included. Luk-I consists of LukS-I and LukF-I. From the deduced amino acid sequence of LukS-I, a 4-residue sequence, K135K136I137S138, at the root of the stem region was found to be identical with that of the phosphorylated segment of a protein phosphorylated by protein kinase A. A mutant of LukS-I (MLSI-SA), in which the Ser138 residue was replaced by an alanine residue, was created, purified, and assayed for its leukocytolytic and pore-forming activities with LukF-I. Both LukS-I and MLSI-SA formed a ring-shaped complex with LukF-I on rabbit erythrocytes and human polymorphonuclear leukocytes (HPMNLs) membrane. However, MLSI-SA showed no leukocytolytic activity with LukF-I. LukS-I was phosphorylated by protein kinase A in the presence of [gamma-P-32] ATP in a cell-free system, but MLSI-SA was not phosphorylated significantly. A potent and selective inhibitor of protein kinase A (N- [2(p-bromocinnamylamino) ethyl]-5-isoquinolinesulfonamide (H-89)) showed 50% inhibition of the Luk-I-induced cell lysis at 0.5 nm. Thus, it is concluded that the phosphorylation of the Ser138 residue in the 4-residue segment K135K136I137S138 of LukS-I is important for the leukocytolysis of HPMNLs.