MicroRNAs (miRNAs) are a class of small, non-coding RNAs that posttranscriptionally regulate gene expression and thereby control most, if not all, biological processes. Aberrant miRNA expression has been linked to a variety of human diseases including cancer, but the underlying molecular mechanism often remains unclear. Here we have screened a miRNA expression library in a growth factor-dependent mouse pre-B-cell system to identify miRNAs with oncogenic activity. We show that miR-125b is sufficient to render pre-B cells growth factor independent and demonstrate that continuous expression of miR-125b is necessary to keep these cells in a transformed state. Mechanistically, we find that the expression of miR-125b protects against apoptosis induced by growth factor withdrawal, and that it blocks the differentiation of pre-B to immature B cells. In consequence, miR-125b-transformed cells maintain expression of their pre-B-cell receptor that provides signals for continuous proliferation and survival even in the absence of growth factor. Employing microarray analysis, we identified numerous targets of miR-125b, but only reconstitution of MAP3K11, a critical regulator of mitogen- and stress-activated kinase signaling, interferes with the cellular fitness of the transformed cells. Together, this indicates that MAP3K11 might function as an important tumor suppressor neutralized by oncomiR-125b in B-cell leukemia.