We used somatic cell nuclear transfer (SCNT) to generate a mouse from the nucleus of an IgG1⁺ ovalbumin-specific B cell. The resulting OBI mice show generally normal B-cell development, with elevated percentages of marginal zone B cells and a reduction in B-1 B cells. Whereas OBI RAG1^−/− mice have exclusively IgG1 anti-ovalbumin in their serum, OBI mice show elevated levels of anti-ovalbumin of nearly all isotypes 3′ of the γ1 constant region in the IgH locus, indicating that class switch recombination (CSR) occurs in the absence of immunization with ovalbumin. This CSR is associated with the presence of IgM⁺IgG1⁺ double producer B cells that represent <1% of total B cells, accumulate in the peritoneal cavity, and account for near-normal levels of serum IgM and IgG3.