The cellular signals required for induction of immunoglobulin (Ig) class switching are only partially understood. Two processes that are considered to be necessary for such induction are DNA synthesis and germline constant heavy (CH) gene transcription. We now show that an additional signal, as mediated by interleukin 5 (IL-5), is also required. To induce proliferation of resting B cells, but not Ig secretion, we utilized anti-IgD antibodies conjugated to dextran (alpha delta-dex). The addition of IL-4, a well-established switch factor for the IgG1 subclass, to alpha delta-dex-activated cell cultures failed to induce IgG1 secretion or mIgG1+ cells unless IL-5 was also present. While IL-4 stimulated an increase in germline gamma 1 RNA in alpha delta-dex-activated cells, this effect could neither be induced nor enhanced by IL-5. By contrast, IL-5 strongly enhanced steady-state levels of productive gamma 1 RNA induced by alpha delta-dex and IL-4, suggesting that IL-5 stimulated IgG1 switch rearrangement. To test this possibility we measured switch (S) mu-S gamma 1 DNA recombination events using a newly developed assay, digestion circularization polymerase chain reaction (DC-PCR). We demonstrated that IL-5 was necessary for induction of S mu-S gamma 1 DNA rearrangement in alpha delta-dex plus IL-4-activated cells but that it had little effect on rearrangement in the absence of IL-4. Our data strongly suggest, therefore, a three-component model for induction of Ig class switching. This model includes germline CH gene transcription, DNA synthesis, and a third component that is necessary for recombination.