We have isolated two cationic peptides, sharing partial homology with each other, from the venom of South African scorpions. Both synthetic peptides-one containing 44 amino acids, the other containing 45 amino acids-were constructed. At submicromolar concentrations they can activate granulocytes as evidenced by a concentration dependent chemotaxis and exocytosis. They also strongly inhibit the production of superoxide anions. At higher concentrations they act as pore formers and induce leakage of the cells. These different effects may be related to their amphipathic structure.