Oral treatment with cephalexin resulted in bacteriologic cure in 24 of 25 (96%) dogs with urinary tract infection caused by Klebsiella pneumoniae. All 25 of the isolates of K pneumoniae were susceptible in vitro to cephalexin at a concentration of less than or equal to 64 micrograms/ml, well below the mean concentration of cephalexin attained in the urine of clinically normal dogs given similar doses of this drug.