Environmental sensing in bacteria often involves the concerted action of sensor kinases and response regulators. Degenerate oligonucleotide primers were designed on the basis of amino acid similarity in the response regulators of these two-component sytems. The primers were used in PCR to specifically amplify an internal DNA segment corresponding to the receiver module domain from genes encoding response regulators. Amplification products of the expected size were obtained from 12 different Gram-positive and Gram-negative bacteria. Sequence analysis revealed that 22 DNA fragments, which clearly originated from response regulator genes, were amplified from Escherichia coli, Agrobacterium tumefaciens, Bacillus subtilis and Lactobacillus bulgaricus. In each of these four species the receiver module of putative response regulator genes, which do not seem to be related to any of the already characterized genes, was identified. This simple and powerful method is therefore particularly useful for discovering new signal transduction systems which cannot be revealed by usual genetic studies.