SummaryStre pto b a c illus m o nil ifo rm is is a Gram-negat ive bac terium found in various laboratory anim al species and is the cause of rat bit e fever and Haverhill fever in man. In order to evaluate a polymerase chain reaction (PCR ) for the detection of this zoonotic bact erium in anim al tissues a set of primers was designed based on the DNA base sequence of part of the 16S rRNA gene from 11 S. m o nili fo rm is strains. T he PCR detected as few as 2±6 copies of S. m o nilifo rm is DNA. A 296 bp DNA fragm ent was ampli®ed from S. m o nilifo rm is strains from rodents, humans and turkeys. Amplicons of about the same size were obtained from T he PCR detected S. m o ni lifo rm isinfection in all four orally-and four intravenously-infected C57BL/6 mice and the bac terium was cultured from all but one mouse. T he PCR detected S. m o nilifo rm is infection in all 12 orally-infected WU rats, and in ®ve of eight rats exposed to natural infection. Enzyme linked immunosorbent assay (ELISA) and PCR were equally successful in detecting infection in rats but S. m o ni lifo rm is was not detected by using culture. Keywords S. m o nilifo rm is; PCR; mouse; rat; health monitoring Stre pto b a c illus m o nil ifo rm isis a Gramnegative bac terium that can occur in various laboratory animal species including gerbils, guineapigs, mice and rats. Humans may become infected through ingestion of contam inated water or through a bit e or a scratch of an animal, leading to Haverhill fever (HF) and rat bite fever (RBF), respectively (Wullenweber 1995). It is commonly believed that with the use of modern maintenance system s streptobac illosis has been eradicated from laboratory animal colonies. T here are however recent reports on the isolation of S. m o nilifo rm is from laboratory rodents (Wullenweber e t a l. 1990, Koopman e t a l. 1991, Kirchner e t a l. 1992, Wullenweber e t a l. 1992, Glastonbury e t a l. 1996 ), and serological observations using an ELISA (Boot e t a l. 1993 ) suggest that latent infection may be common in rats (unpublished observations). As the culture of S. m o ni lifo rm is from healthy animals is dif®cult, we developed and evaluated a PCR for the detection of the bacterium in tissues of mice and rats. Materials and methods Mic ro o rga nism sSe ve nte e n S. m o nilifo rm is strains were obtai ned from mice (3 ), rats (3 ), a spinifex hopping mouse (1 ), turkeys (4 ) and from cases of HF (1 ) and RBF (5 ) in humans (Table 1).