The authors observed an immunostimulating effect of levamisole and MDP on fagocyting activity (FA) of neutrophile polymorphonuclear leucocytes in dependence on ageing in rats. They observed 3 age groups, namely 3, 12 and 18 month old animals. The substances were administered during a period of 30 days. It was found out that following both substances, FA elevation in 3 month old animals was significant after 15 days of medicament administration, though, in the 12 month old animals it supervened after 15 and also 30 days. In 18 month old animals following levamisole administration, a significant FA increase supervened only following 15 days; though after MDP administration, it supervened after 25 and also 30 days. It is remarkable that in this group, during administration of both medicaments, 38-50% of animals died. The study points out that age represents a factor which modifies the effect of immunostimulating substances. (Fig. 2, Ref. 16)