Early investigations of lymphocyte migration in the rat operationally identified a lymphocyte membrane protein, designated 'A.11', which mediates lymphocyte adherence to lymph node (LN) high endothelial venules (HEV). To determine the primary structure of A.11 and examine its expression in lymphoid cells, we constructed an expression phage cDNA library of rat thoracic duct lymphocytes (TDL) and performed screening by immunoselection (utilizing an anti-A.11 polyclonal antiserum) as well as by hybridization selection. We have isolated a ~1.6kb clone, RS-2, and sequencing revealed that it encodes rat L-selectin. The clone contains the complete coding sequence, a 105-bp 5' untranslated region and a 359-bp 3' untranslated region. Transfection of RS-2 cDNA into 70Z/3 cells conferred binding to HEV concomitant with expression of A.11, providing direct evidence that A.11 is rat L-selectin. Metabolic radiolabelling studies revealed that thymocytes synthesize markedly less L-selectin than do TDL or LN lymphocytes. However, Northern blot studies using RS-2 as a probe indicate that thymocytes possess more L-selectin RNA than does TDL. Together, these data provide evidence that post-transcriptional events contribute to regulation of L-selectin expression in thymocytes.