The interaction of BKV with its natural target cells, human kidney epithelial cells, has not been studied. In vitro infections of human primary kidney epithelial cells were performed to investigate a BKV infection in its natural host cell. BKV undergoes a lytic replication cycle in this system: high levels of T antigen expression were first detected at 36 h postinfection, while viral DNA replication, capsid protein expression, and progeny virus were observed at 48 h postinfection. It was observed that the related polyomavirus SV40 is incapable of infecting human kidney epithelium except in the presence of the GM1 ganglioside, recently reported to be an SV40 receptor.