Deregulated expression of the Myc family of transcription factors (c-, N-, and L-myc) contributes to the development of many cancers by a mechanism believed to involve the stimulation of cell proliferation and inhibition of differentiation. However, using B cell-specific c-/N-myc double-knockout mice and E(mu)-myc transgenic mice bred onto genetic backgrounds (recombinase-activating gene 2-/- and Btk-/- Tec-/-) whereby B cell development is arrested, we show that Myc is necessary to stimulate both proliferation and differentiation in primary B cells. Moreover, Myc expression results in sustained increases in intracellular Ca2+ ([Ca2+]i), which is required for Myc to stimulate B cell proliferation and differentiation. The increase in [Ca2+]i correlates with constitutive nuclear factor of activated T cells (NFAT) nuclear translocation, reduced Ca2+ efflux, and decreased expression of the plasma membrane Ca2+-adenosine triphosphatase (PMCA) efflux pump. Our findings demonstrate a revised model whereby Myc promotes both proliferation and differentiation, in part by a remarkable mechanism whereby Myc amplifies Ca2+ signals, thereby enabling the concurrent expression of Myc- and Ca2+-regulated target genes.