Pertussigen, a protein toxin purified from Bordetella pertussis, and fucoidin, a high molecular weight sulfated polysaccharide, were analyzed for their ability to inhibit lymphocyte recirculation in vivo. Pertussigen treatment of lymphocytes resulted in a dosage- and time-dependent loss of their ability to localize in lymph nodes or Peyer's patches. This toxin-induced alteration did not reverse after extended lymphocyte culture in toxin-free media, and had no effect on lymphocyte viability or activation by mitogens. Furthermore, pertussigen-treated lymphocytes retained the ability to specifically adhere to high endothelial cells in an in vitro binding assay. Kinetic studies suggested that the toxin's molecular action on lymphocytes is analogous to that reported for pancreatic islets and hormone-responsive cultured cell lines. Inhibition of lymphocyte recirculation by fucoidin was also observed in vivo. Fucoidin-mediated inhibition of lymphocyte localization to peripheral lymph nodes was reversible with time, and could not be effected by pretreatment of lymphocytes with the polysaccharide. Furthermore, we confirmed the observation that fucoidin blocks lymphocyte adhesion to high endothelial cells in vitro. On the basis of these observations, we propose that the mechanism of lymphocyte extravasation involves a specific receptor-mediated binding event followed by an adenylate cyclase-dependent activation of cell motility. Fucoidin is capable of interfering with the primary adhesion event, whereas pertussigen selectively inhibits the second process to block lymphocyte recirculation in vivo.