Resting murine B cells activated with bacterial LPS co-express membrane (m)IgG1 and mIgE upon stimulation with IL-4. In this report, we combine both cellular and molecular approaches to elucidate the mechanism underlying this co-expression. We demonstrate that an anti-IgG1 antibody specifically and selectively inhibits IgE secretion (approximately 70%) by LPS + IL-4-stimulated B cells, which provides functional evidence for mIgG1 expression by precursors of IgE-secreting cells. The IgG1 and IgE secretory responses are separated temporally by approximately 16 h, with IgE production developing later than IgG1. A similar delay is observed in the appearance of mIgE+ cells suggesting that class switching to IgG1 precedes that to IgE. In the sort-purified, mIgG1+mIgE+ B cell population approximately 25% of cells expressed cytoplasmic (c) (secretory) IgG1 and approximately 15% expressed cIgE at the time of their isolation. However, only a small percent of the mIgG1+mIgE+ cells co-expressed cIgG1 and cIgE, further suggesting a temporal separation in IgG1 and IgE secretion within individual cells, but indicating that single cells can co-secrete these two Ig isotypes. Furthermore, the absolute level and rate of increase of IgG1 secretion by mIgG1+mIgE+ cells, upon their isolation and reculture, is lower than that for mIgG1+mIgE- cells suggesting a loss of CH gamma 1 expression in the former population. Analysis of total, unselected circular DNA excision products in LPS + IL-4-activated B cells demonstrates that most, if not all, of the DNA encoding the IgG1 constant heavy gene (CH gamma 1) (i.e., products of a class switch to IgE) have been rearranged. Collectively this data provides strong evidence at both the cellular and molecular level that the predominant mode of switching to IgE in response to in vitro stimulation by LPS + IL-4 is from IgM to IgG1 to IgE.