BACKGROUND: Porcine endogenous retroviruses (PERVs) form part of the chromosomes of all pigs. Since they can be produced as infectious virion and infect human cells, safety issues on PERVs infection to human are still controversial and is one of main hurdles of xenotransplantation using pig cells or organs. It has been reported that the established porcine cell line, PK-15, produces PERVs and can infect the human cell lines. Therefore, clonal analysis on human cell line infected with PERV is a prerequisite to characterize the infectivity to human cells and to investigate the harmfulness of PERVs to human. MATERIALS AND METHODS: For the characterization of PERV that originates from porcine cell line, PK-15, full length PERV cloning from genomic DNA of PK-15 was performed and partial sequences of both ends were achieved. Cell clones from human cell line, 293, persistently infected with PERVs from PK-15 were established by the method of limiting dilution. Nested PCR and direct sequencing of PCR products in each clone were carried out so as to confirm the PERV genomes in each clone. The growth rate of each clone was checked using cell counting and 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT) assay, the infectivity by reverse transcriptase (RT) assay, and genetic analysis by karyotyping. RESULTS: A total of 12 genomic PERV clones could be retrieved; 1 with full length, 4 with defective forms, and others with irrelevant sequences. Intact PERV was thought to be able to infect 293 and the PERV-infected cell clones were selected by limiting dilution. PCR results confirmed that nine cell clones were infected with PERV, and sequence alignment data on PCR products of pol region from PK-15 and human cell clones with PERV showed very similar results. Cell counting and MTT assay for growth kinetics of each clone indicated that two clones showed reduced growth rate. However, it was difficult to verify the effect of PERV infection on the cell growth because of the presence of many genetic alterations in 293 parental cells. No RT activities were detected in the culture supernatant from PERV-infected 293 cell clones. CONCLUSION: The sequences of PERVs were detected in human cell clones after PERV infection, but PERV virions could not be detected from the culture supernatant by RT assay.