The Myb transcription factors, c-Myb, A-Myb, and B-Myb, regulate cell differentiation and/or proliferation. To investigate the role of B-Myb in embryogenesis, we introduced an inducible dominant interfering Myb protein (MERT) into embryonic stem (ES) cells, which express B-Myb as an exclusive member of Myb family. Disruption of normal B-Myb function by the conditional activation of MERT caused a drastic morphological alteration of ES cells and G(1)-S cell cycle arrest. The inhibition of B-Myb function by MERT dissociated tightly packed ES cell colonies into dispersed single cells that subsequently detached from the culture dish. Cell adhesion analyses revealed that suppression of B-Myb function reduced the adhesion with extracellular matrix proteins, such as laminin, collagen, and fibronectin. This reduction was presumably due to decreased cell surface expression of beta1 integrin. Embryoid body formation was also severely retarded by the activation of MERT. This impairment was attributed to reduced expression of E-cadherin, which functions as a homophilic intercellular adhesion molecule. Simultaneously, blocking B-Myb function did not alter the expression of differentiation markers. Our data indicate that B-Myb plays important roles in regulating cell adhesion and cell cycle progression. These results are well consistent with the recent report on the phenotype of B-Myb null mice and show that the regulation of cell adhesion is an important B-Myb function that has not yet been assumed.