Seven nylon membranes were tested in different DNA transfer and post–transfer procedures. The sensitivity of the hybridization signals was increased ten–fold over conventional Southern blot analysis using nitrocellulose, by UV–irradiating DNA blotted to nylon membranes at high salt concentrations. This treatment also improved the sensitivity of the signals with nitrocellulose membranes. UV–irradiation of DNA transferred to nylon membranes in low salt buffers or alkali reduced hybridization efficiency.