Germline C_H transcripts initiate from a non-coding I exon and terminate downstream of the associated C_H exons. Ig heavy chain class switch recombination from the VDJ - C_μ gene to particular downstream C_H genes appears to be regulated by a process that involves mltogen and/or cytokine induction of germline C_H transcripts from the downstream genes. We have examined the expression of germline C_μ transcripts (I_μ-C_μ transcripts) in splenic B cells and pre-B cell lines after cytokine and mltogen stimulation. In contrast to the expression of the germline transcripts from downstream C_H genes, expression of germline C_μ transcripts was constitutive and unaffected by mltogen and cytokine treatment. After a primary switch recombination event, the germline I_μ promoter, which is now associated with a downstream C_H gene, continues to be active—leading to the generation of a novel germline transcript consisting of the I_μ exon spliced to the C_H exons of the switched C_H gene. We discuss the potential role of the expression of hybrid I_μ-contalning transcripts in the class switch process. We also describe a novel and sensitive assay, based on the detection of the hybrid I_μ-contalnlng transcripts, that allows detection of class switch recombination events even in heterogeneous populations of cells.