Germinal centers (GCs) represent the main sites for the generation of high-affinity, class-switched antibodies during T cell-dependent antibody responses. To study gene function specifically in GC B cells, we generated Cgamma1-cre mice in which the expression of Cre recombinase is induced by transcription of the Ig gamma1 constant region gene segment (Cgamma1). In these mice, Cre-mediated recombination at the fas, Igbeta, IgH, and Rosa26 loci occurred in GC B cells as early as 4 days after immunization with T cell-dependent antigens and involved >85% of GC B cells at the peak of the GC reaction. Less than 2% of IgM(+) B cells showed Cre-mediated recombination. These cells carried few Ig somatic mutations, expressed germ-line Cgamma1- and activation-induced cytidine deaminase-specific transcripts and likely include GC B cell founders and/or plasma cell precursors. Cre-mediated recombination involved most IgG1, but also a fraction of IgG3-, IgG2a-, IgG2b-, and IgA-expressing GC and post-GC B cells. This result indicates that a GC B cell can transcribe more than one downstream C(H) gene before undergoing class switch recombination. The efficient induction of Cre expression in GC B cells makes the Cgamma1-cre allele a powerful tool for the genetic analysis of these cells, as well as, in combination with a suitable marker for Cre-mediated recombination, the tracking of class-switched memory B and plasma cells in vivo. To expedite the genetic analysis of GC B cells, we have established Cgamma1-cre F(1) embryonic stem cells, allowing further rounds of gene targeting and the cloning of compound mutants by tetraploid embryo complementation.