An attempt to isolate Enterobacter cloacae, Serratia marcescens and Yersinia enterocolitica was made in a total of 931 laboratory animals (mice, rats and rabbits) from five representative commercial breeders in this country and from our laboratory colonies. E. cloacae organisms were isolated from feces and other specimens of 15 mice and 17 rats between November 1979 and July 1980. Neither S. marcescens nor Y. enterocolitica was detectable in any specimen of feces, fur and skin, nasal discharge, lungs, urinary bladder or urine from the animals examined.