Intermediate lymph was collected from the efferent duct of a single, superficial lymph node in unanaesthetised sheep for a few days before and after they were given an intravenous (i.v.) injection of a sulphated polysaccharide, fucoidan, at a dose of 10 mg per kg. Within an hour or so of the injection, the recirculation of lymphocytes through the node under study fell by over 80% and remained low for a few hours. This acute reduction in lymphocyte recirculation could be duplicated almost exactly by giving an i.v. dose (250 micrograms per kg) of a synthetic corticosteroid, dexamethasone, instead of fucoidan. Also, during the fall in lymphocyte recirculation that followed the injection of fucoidan, the concentration of cortisol in the blood plasma increased nearly 10-fold. These findings strongly suggest that much of the effect of exogenous sulphated polysaccharides on the recirculation of lymphocytes is neither direct nor specific, but secondary to an increased secretion of corticosteroid hormones.