During the period from January to March 1995, the authors first isolated Bartonella henselae from the blood of three (9.1%) of 33 domestic cats in Japan. The three cats were a 1.5-year male pet cat-old with urinary retention, and 6-year-old female pound and age-unknown female pet cats with no abnormalities. The blood was taken in a lysis-centrifugation tube (Wampole Isolator tube) and cultured on 5% rabbit-blood heart infusion agar plates at 35 degrees C in the 5% CO2 atmosphere. Visible tiny rough colonies developed 14 days after incubation. The isolates showed Gram-negative and pleomorphic rods in microscopic observation. The DNA extracted from the isolates was amplified by PCR using two primers, which were specific for the rikettsial citrate synthase gene. The isolates were identified as B. henselae from the patterns of digestion with TaqI and HhaI of the amplified gene. It was confirmed that cats in Japan harbored B. henselae in their blood, and that cats play a significant role as the reservoir of the organism.