DNA synthesis and DNA polymerase activities were followed in FL cell cultures (clone 5.86) at different stages of differentiation. A temporary block of growth and DNA synthesis was observed after the addition of the inducers (DMSO or HMBA). This delay in the growth and in the DNA synthesis initiation is not observed in cultures of DMSO-resistant variants after treatment with DMSO. In both uninduced and induced cultures, during growth, the DNA gamma-polymerase activity is constant and the alpha-polymerase activity is strictly dependent on the DNA synthesis rate. On the other hand, a different behaviour between induced and uninduced cultures is observed for the DNA polymerase beta: its activity is constant in uninduced cultures, whereas it changes in induced cultures at various stages of differentiation, dropping to lower values at early times and rising to values similar to those observed in uninduced cultures at later times. This behaviour is not observed in cultures of a DMSO-resistant variant clone: in this case the beta-polymerase activity is constant in both the absence and the presence of DMSO or HMBA.