A method is described for the lysis of bacterial cells grown on nitrocellulose filters which allows the use of nonradioactive (biotin labeled) probes in colony hybridization. Used in conjunction with a colorimetric assay involving streptavidin and alkaline phosphatase this lysis method allows the detection of clones containing a target nucleic acid sequence. Sites of positive hybridization produce dark blue-purple signals, while nonreacting clones give very light blue signals. The occurrence of false-positive and background signals is minimal. With pBR322 as the target sequence it was possible to detect approximately 20 clones in the presence of 10⁵ plasmid-free cells. It was also possible to detect low-frequency plasmid-free cells in a population of clones containing the target sequence.