There is increasing use of polymerase chain reaction techniques to diagnose infection. We report the use of polymerase chain reaction using a generic section of the bacterial 16S rDNA gene--followed by nucleotide sequencing--to determine the species of the infecting bacteria. In the first case, the clinical and microbiological diagnosis of meningococcal septicaemia was in agreement with the results from polymerase chain reaction technique. In the second case, a Yersinia enterocolitica bacteremia was detected by the polymerase chain reaction technique, but missed with conventional blood culture techniques.