The single-stranded DNA binding activity of the Escherichia coli RecA protein is crucial for homologous recombination to occur. This and other biochemical activities of ssDNA binding proteins may be affected by various factors. In this study, we analyzed the effect of CaCl(2), NaCl and NH(4)NO(3) salts in combination with the pH and nucleotide cofactor effect on the ssDNA-binding activity of RecA. The studies revealed that, in addition to the inhibitory effect, these salts exert also a stimulatory effect on RecA. These effects occur only under very strict conditions, and the presence or absence and the type of nucleotide cofactor play here a major role. It was observed that in contrast to ATP, ATPγS prevented the inhibitory effect of NaCl and NH(4)NO(3), even at very high salt concentration. These results indicate that ATPγS most likely stabilizes the structure of RecA required for DNA binding, making it resistant to high salt concentrations.