During embryogenesis, colonization of the thymic rudiment by hemopoietic progenitor cells depends on the adhesion of these cells to the jugular endothelium. Previously, we showed that progenitor T cells (pro-T cells) interact with alpha 6 integrins present on vascular endothelium. Here, we demonstrate that anti-alpha 6 integrin antibodies reduced the number of thymocytes up to 80% in a congenic mouse model for thymus colonization by pro-T cells. In organotypic thymus cultures, the anti-alpha 6 integrin antibodies did not influence T cell development and proliferation. From this, we conclude that alpha 6 integrin participates in thymus homing. During mouse thymus ontogeny, alpha 6 integrin mRNA and protein expression was found as early as day 10 of development; at day 11, perithymic endothelial cells were alpha 6 integrin positive. Two alpha 6 integrin mRNA exist which are produced by alternative exon usage. The longer form, alpha 6 integrin, predominates during early embryonic stages, while the shorter alpha 6A form was present later during development. Although alpha 6 integrins can be displayed by immature thymocytes, strongest expression was found on intra- and perithymic vascular endothelium. These data suggest that alpha 6 integrins are involved in the homing of pro-T cells to the developing thymus by mediating adhesion of pro-T cells to the vascular endothelium.