DnaK is known to be highly conserved in all species and is a major immunogen in Streptococcus pneumoniae. To elucidate the role of dnaK in S. pneumoniae, dnaK was cloned in Escherichia coli using a homologous dnaK probe generated by PCR. The His-tagged DnaK was overexpressed in soluble form and purified from E. coli. Alignment of the deduced DnaK amino acid sequence from nucleotide sequences of the cloned dnaK revealed high homology with DnaK analogs in E. coli (53%) and Staphylococcus aureus (73%). However, anti-pneumococcal DnaK antiserum did not crossreact with DnaK analogs in E. coli, S. aureus and human cells suggesting that pneumococcal DnaK might be a good candidate as a vaccine. z 1998 Federation of European Microbiological Societies. Published by Elsevier Science B.V.