Human immunodeficiency virus type 1 (HIV-1) is one of the lentiviruses, and unlike other retroviruses, HIV-1 is capable of infecting not only dividing cells but also non-dividing cells. It has been strongly suggested that this property is due to the viral mechanism which facilitates the integration of the viral genome cDNA into the host chromosome more efficiently than other retroviruses. HIV-1 integrase (IN) is a viral protein which catalyzes insertion of the viral genome cDNA into the host cell chromosome. However, it has been suggested that HIV-1 IN also plays putative roles at the steps prior to integration such as uncoating, reverse transcription and nuclear transport of the viral cDNA. In this study, we tried to identify the novel host factor which interacted with HIV-1 IN using two different kinds of yeast two hybrid methods: the conventional yeast two hybrid method and the mating method. First, the full-length cDNA fragment of HIV-1 IN was amplified using polymerase chain reaction (PCR), and the amplified products were ligated into pGBT 9 vector as bait. Plasmid vectors expressing human lymphocytes cDNA library and HeLa cDNA library were used as prey plasmid in the conventional yeast two hybrid method and the mating method, respectively. These plasmids were transformed into the corresponding yeast strain cells, and several positive clones were isolated. As a result, a known gene product was identified as a candidate. Further analysis revealed that this protein was expressed in HeLa, 293 T cells, primary macrophages and activated T lymphocytes, and that suppression of this protein expression affected HIV-1 replication.