Histological and immunocytochemical investigations were performed on different organs (brain, duodenum and thymus) of mice following lethal (420 mug/kg) or sublethal (10 mug/kg) intraperitoneal injection of yessotoxin (YTX). No morpho-functional modifications were observed in large neurons of the cerebral and cerebellar cortex with the sub-lethal dose, nor in the cerebral cortex with the lethal dose. The duodenum also did not show significant alterations. However, there was an inflammation response to the toxin, in which blood cells and cytokines were involved. This was more evident with the lethal YTX dose. The thymus and, in general, the immune system are the main targets of YTX at both the concentrations used. Furthermore, the alterations present in the thymus may support tumorigenic implications. (C) 2004 Elsevier Ltd. All rights reserved.