The E2A gene encodes two E protein/class I basic helix-loop-helix transcription factors, E12 and E47, that are essential for B lymphopoiesis. In addition to the DNA-binding and protein dimerization domain, the E proteins share two highly conserved transcription activation domains. In this study, we show that both activation domains are required for optimal E2A-dependent transcription. Surprisingly, however, neither activation domain is required for E2A to rescue B lymphopoiesis from E2A(-/-) hemopoietic progenitors, although the N terminus of E2A, which harbors some transcription capacity, is required. Therefore, the E protein activation domains function redundantly in promoting B cell development. In contrast, the N-terminal activation domain, AD1, is required for a newly described ability of E2A to suppress macrophage development in vitro. Our findings demonstrate distinct functionalities for the E protein activation domains in B lymphocytes and macrophages.