Integrins comprise a superfamily of alpha beta heterodimers that serve as cell signaling as well as adhesion molecules. We demonstrate that the alpha 3 beta 1 and alpha 6 beta 1 integrins are laminin/merosin receptors expressed in human thymocytes. By reverse transcriptase-PCR analysis, we determined that the alpha 3A beta 1, but not the alpha 3B beta 1, cytoplasmic structural variant of alpha 3 beta 1 is expressed in thymocytes. In contrast, both alpha 6A beta 1 and alpha 6B beta 1 cytoplasmic structural variants of alpha 6 beta 1 are expressed. A small percentage (10 to 15%) of human thymocytes bind to immobilized laminin, and even fewer (3 to 5%) bind to merosin, the laminin isoform normally present in the thymus. This binding, however, can be increased to 39 to 41% after activation of thymocytes with Mn2+ (or PMA). Binding to either laminin or merosin is completely inhibited by anti-beta 1 mAb or by a mixture of anti-alpha 3 and anti-alpha 6 mAbs, indicating that both alpha 3 beta 1 and alpha 6 beta 1 participate in thymocyte adhesion to the laminin family of extracellular matrix proteins. The protein kinase C inhibitors, calphostin C and staurosporine, inhibit Mn(2+)-enhanced thymocyte binding, suggesting that protein kinase C activity is crucial for the binding. Furthermore, the data indicate that at least two divalent cation binding sites serve to regulate integrin binding activity. Finally, we show that both immobilized laminin and merosin have costimulatory function for anti-CD3-induced thymocyte proliferation, and both anti-alpha 3 and anti-alpha 6 mAbs can block this proliferative response. The cooperative function of alpha 3 beta 1 and alpha 6 beta 1 evidenced in the laminin/merosin binding and proliferation assays suggests that thymocyte-merosin interactions may play an important role in thymic T cell development.