The basic defect in cystic fibrosis is the chloride impermeability of the plasmalemm in different cells. A candidate for the chloride channel, thought to be affected in the syndrome, is “Porin 31HL” recently described by us. The molecule is i) expressed in the plasmalemm of different cells, it has ii) a molecular mass of 31000 Daltons, it shows iii) high conductance in artificial membranes and it can be iv) modified by 4,4′-Diisothiocyana-tostilben-2,2′-disulfonat. A porin in the outer membrane of cells should furthermore v) be regulated by modulators. All these characters of “Porin 31HL” correspond to those given in literature for chloride channels. The regulation of the channels can be explained by a two component flip flop model.