- In a previous study we have shown that a single infectious particle of Moloney murine leukaemia virus per cell is sufficient to facilitate chemical carcinogenesis in normal rat kidney cells. When these cells are exposed to the carcinogen after a low number of passages post-infection (p.i.), cell transformation becomes apparent only after many subsequent passages. On the other hand, when exposure is done after a high number of passages p.i., cell transformation can be detected in the treated culture or at the next passage. It is thus evident that whereas the carcinogenic effect is rapid, the viral effect becomes apparent only after a long period of latency. Here we provide evidence that this viral effect requires multiple proviruses and that the long latent period reflects the time needed for a sufficient accumulation of proviruses in some of the cells. This accumulation may result from multiple rounds of superinfection by virions released into the culture medium, although we cannot exclude other mechanisms of provirus amplification. Our data also suggest that this amplification enhances virus production.