In mammals, interaction of CD28 with CD80 or CD86 molecules provides costimulatory signals for T cell activation. Such signals lead to increased gene and protein expression of IL-2 by activated T-cells. CD80 and CD86 have only been cloned in mammals and birds thus far, and their functional characterization is basically limited to their study in mammals. Thus, in the present study we show for the first time the cloning of a rainbow trout molecule (rtCD80/86) that shows a high degree of homology to CD80 and CD86 molecules. In addition, we have identified and cloned trout IL-2 and we have shown that recombinantly produced trout CD80/86 upregulates the expression of IL-2 in trout blood leukocytes. This finding indicates that this capacity to modulate IL-2 expression is a primordial function that has been conserved both in fish and mammalian CD80/CD86 molecules throughout evolutionary time.