An amplification product that occurred in negative controls of a PCR using a primer system for Legionella 55 ribosomal RNA was characterized by direct sequencing. The amplification product did not hybridize to a Legionella specific oligonucleotide. It was derived from bacterial DNA contaminating Taq DNA polymerase, a phenomenon that was previously reported for amplification reactions with universal primer sets for bacterial 16S rRNA. The sequence of the 5S ribosomal fragment had close homology to the 5S-rRNA of the species Pseudomonas fluorescens, Pseudomonas aeruginosa, Alcaligenes faecalis, and Azotobacter vinelandii. These findings confirm that the DNA contaminations in Taq DNA polymerase belong to other species than Thermus aquaticus or Escherichia coli.