Heptaminol AMP amidate (HAA), a newly developed nucleotide derivative, was found to restore the immunosuppression in mice due to the induction of suppressor T (Ts) cells by concanavalin A (Con A) (50 micrograms/body). HAA also inhibited Con A-mediated in vitro induction of Ts cells. On the contrary, the administration of HAA in mice primed with keyhole lympet hemocyanin (KLH) (30 micrograms/body) caused an enhanced induction of antigen specific helper T (Th) cells. Effects of HAA on Ts and Th cells were found to be dependent on their level of induction. The administration of HAA also increased the spleen cell number and augmented the plaque forming cell response to some extent in cyclophosphamide treated mice. The present results suggested that HAA-mediated immunopotentiation was possible by a combined suppressive effect on Ts cells and enhancing effect on Th cells.