Deoxyspergualin (DSG), which was discovered to be an immunosuppressive agent, was examined for its in vivo effect on parasites of rodent malaria. Although the mice that were not treated by DSG had an increased parasite percentage (% parasitemia) until they died, those that were treated with DSG had a decreased parasitemia and finally had 0% parasites. The spleens of infected mice became small by DSG treatment. Parasitemia of mice increased again after DSG treatment was stopped. However, DSG was a polyamine inhibitor. The two other types of polyamine inhibitors used in this study were not effective for decreasing the % parasitemia of Plasmodium berghei. Only DSG was available and the survival time of mice increased. The antiprotozoal effects shown by DSG – even this chemical is an immunosuppressive agent – suggest that there is a relation between the inhibition of the polyamine synthetic pathway or immunosuppression of DSG and the suppressive effect of malarial parasi