Previous studies have indicated that transcription of germ-line (GL) CH genes is necessary to obtain immunoglobulin (Ig) class switching. We report here a correlation between proliferation, switching and GL transcripts. Sμ-Sγ1 switch recombination in lipopolysaccharide (LPS) + interleukin-4 (IL-4)-activated mouse B cells was assayed by a digestion-circularization polymerase chain reaction. Switching to γ1 is reduced upon inhibition of DNA synthesis with hydroxyurea (HU) or aphidicholin (AC). Incubation of activated B cells with HU severely reduces steady-state levels of GL γ1 and ϵ RNA. By utilizing elutriation to synchronize B cell blasts in different phases of the cell cycle, it was found that GL γ1 transcripts are mainly expressed in G1 and S phases, but not in G0. Using the electrophoretic mobility shift assay, we characterized two major LPS-induced complexes, which bind to the GL γ1 promoter and are expressed at levels which correlate with the amount of LPS-induced DNA synthesis. Furthermore, the intensity of the complexes is reduced when cells are arrested with the DNA synthesis inhibitors HU or AC. Elutriation experiments revealed that the complexes are expressed in G1 and S, but not in G0. They bind to an Ets consensus element near the major initiation sites used in proliferating cells. The possible implications of these findings for Ig isotype switching are discussed.