Recent studies have suggested that Bcl-2 can affect cell cycle re-entry by inhibiting the transition from G(0)/G(1) to S phase. In this study, we have taken a novel route to the study of the relationship between Bcl-2 expression and cell cycle progression. Continuous cultures of pEF (control) and Bcl-2 transfected murine hybridoma cells were operated at a range of dilution rates from 0.8 day(-1) down to 0.2 day(-1). The specific growth rate of the pEF cell line was the same as the dilution rate down to a value of 0.6 day(-1). However, as the dilution rate was reduced stepwise to 0.2 day(-1), the growth rate levelled-off at approximately 0.55 day(-1) and this coincided with a fall in culture viability. By contrast, the specific growth rate of the Bcl-2 transfected cell line followed the dilution rate down to a value of 0.3 day(-1) with high levels of cell survival. At high dilution rates, the cell cycle distributions were very similar for both cell lines. However, the distributions diverged as the dilution rate was reduced and, at a rate of 0.2 day(-1), the percentage of G(1) cells in the Bcl-2 culture was 80%, compared to only 56% in the pEF cell population. This corresponded with a greater extension in the duration of the G(1) phase in the Bcl-2 cells, which was 1.7 days at the lowest dilution rate tested, compared to only 0.6 day for the pEF cell line. The durations of the G(2)/M and S phases remained constant throughout the culture. The maximum doubling time was 1.2 days in the pEF culture compared to 2.3 days in the Bcl-2 culture. Analysis of amino acids, ammonia and lactate concentrations indicated that the observed effects on cell cycle dynamics were probably not due to differences in the culture environment. It is suggested that the expression of Bcl-2 can effect G(1) to S phase transition in continuously cycling cells, but this is only apparent at suboptimal growth rates.