Ig kappa gene expression is controlled by two enhancers, one located within the major intron (Ei) and the other located downstream of C kappa (E3'). Whereas loss of E3' has previously been shown to diminish kappa expression, we show here that a rearranged kappa transgene lacking Ei is well expressed, even at the pre-B cell stage. This suggests that E3' alone might be sufficient to give properly regulated transcription throughout B cell development. Indeed, we show that a transgene composed of a beta-globin reporter linked to E3' is expressed in a B cell-specific manner, becoming activated at the late pro-B to pre-B cell stage but with dramatically enhanced activity on B cell activation. Thus, E3' becomes active as a transcription enhancer at the stage when V kappa-J kappa rearrangement is being initiated and is sufficient to yield an expression pattern in a linked reporter gene similar to that of fully rearranged kappa genes.