gamma-Hemolysins are pore-forming toxins which develop from water-soluble monomers by combining two different 'albeit homologous' proteins. They form oligomeric pores in both cell and model membranes by undergoing a still poorly understood conformational rearrangement in the stem region. The stem is formed by three beta-strands, folded onto the core of the soluble protein and completely extended in the pore. We propose a new model to explain such a process. Seven double-cysteine mutants were developed by inserting one cysteine on the stretch that links the beta-hairpin to the core of the protein and another on different positions along the beta-strands. The membrane bound protein was blocked in a non-lytic state by S-S bond formation. Six mutants were oxidized as inactive intermediates, but became active after adding DTT. These results demonstrate that the stem extension can be temporarily frozen and that the beta-barrel formation occurs by beta-strand concerted step-by-step sliding.