In cultures of the latently Epstein-Barr virus (EBV)-infected Burkitt's lymphoma cell line Raji, the detectable amount of the EBV-encoded latent membrane protein (LMP) is transiently increased after addition of fresh nutrient medium containing foetal calf serum. In the current study, the relative amount of LMP and DNA in Raji cells was determined by biparametric flow cytometry analysis at different times after the addition of fresh medium with 10% foetal calf serum to a dense Raji culture. A transient increase in the proportion of LMP-positive cells was observed during the lag phase of the culture. Subsequently, a subpopulation of cells, which had been arrested in the G0 or G1 phase, simultaneously started to progress through the cell cycle. Neither the amount of LMP in the cells, nor the enhanced expression of LMP, was restricted to a certain phase of the cell cycle. Further analysis revealed that the number of LMP-positive cells proceeding simultaneously from the G1 to the S phase of the cell cycle is about the same as the total number of cells changing phases. These results suggest that LMP expression might be one step in the pathway leading to growth activation of resting cells in cultures of the immortalized Raji cell line.