In our laboratory animal facility, Sendai virus (HVJ) contamination occurred in a negative flow rack used for experimental infection with 4 strains of mouse-adapted influenza virus (Inf.V). Anti-HVJ antibody (Ab) was detected in 35/42 mice in the rack. To specify the strain of Inf.V contaminated with HVJ, experimental infection was performed by using A, B and D strains of Inf.V in each vinyl isolator. Anti-HVJ Ab was detected in all mice infected with A strain at day 28 post-infection. As a result of experimental infection with A strain of Inf.V which was treated with anti-HVJ mouse serum, the virus suspension was determined not to contain HVJ and allowed for experimental use in our facility, Since then, HVJ contamination has not occurred in our facility.