The permeabilization by alpha-helical peptides of nucleated mammalian cells can be monitored by flow cytometry. Ethidium bromide, a non fluorescent and poorly membrane permeant molecule, becomes strongly fluorescent only upon binding to DNA. On this basis, the permeabilization of the plasma membrane of HL60 promyelocytic cells induced by alpha-helical peptides such as melittin, succinylated melittin and anionic peptides derived from the N-terminus of HA2 subunit of the influenza virus hemagglutinin, was measured. Melittin (GIGAVLKVLTTGLPALISWIKRKRQQ-NH2) caused a rapid (< 5 min) and dose-dependent (ED50 = 0.5 microM) permeabilization of HL60 cells at neutral pH, whereas the succinylated derivative induced cell permeabilization only at pH below 4.5 with an ED50 = 18 microM. The permeabilization by the anionic E5CA peptide (GLFEAIAEFIEGGWEGLIEGCA) containing 5 glutamic residues occurred (ED50 = 11 microM) at pH ranging from 6.5 to 6.0; replacing the tryptophan residue in position 14 by a phenylalanine residue decreased by about 1 unit the pH at which membrane permeabilization was effective. The membrane permeabilization activity of the E5CA peptide was reversibly abolished when the peptide was linked to a protein carrier. These results show that alpha-helical peptide-induced membrane permeabilization can be easily monitored by using flow cytometry in the presence of a non permeant dye. This method allows a rapid screening and an efficient mean of selection of peptides suitable to induce membrane permeabilization.