In a search for a host fully permissive for the hamster polyomavirus (HaPV) productive cycle in cell culture, the replication of the viral genome has been assayed in a panel of murine and hamster cell types. These experiments led to the conclusion that hamster cells represent the most permissive host for HaPV DNA replication although some murine cells also permit replication of the viral DNA. A single burst of infectious particles is demonstrable in some replication-competent cells, but the outcome of the infection appears to be clearly host dependent. In one hamster cell line (GD36), the virus can be propagated by successive productive cycles. In other hamster cells, despite a successful initial virus burst following transfection of viral DNA, a block in the production of virus particles seems to prevent the spread of infection. This type of restriction level may play a role in the in vivo host range of HaPV.