Effect of a tree tonic, which was the powder composed of several kinds of herbal medicines and rice bran etc., on the vitality, biomass increment and relative growth rate of sixty seedlings of Japanese black pine (Pinus thunbergii) grown in pots was examined at the experimental farm of Hiroshima University from March to November in 1994.0,1,000 and 3,000 individuals of pinewood nematode were injected into each ten seedlings both treated with and without the tree tonic, respectively. The vitality of seedlings with the tree tonic became up while that without it changed little, regardless of the number of nematoda injected. Biomass increment was significantly larger in seedlings with the tree tonic than in those without it. Mean weight of individuals of seedlings with the tree tonic were evaluated to grow up about twice as large as those without it one year after planting seedlings in pots, based on the data of relative growth rate obtained in both seedlings with and without tree tonic treatments. There was a little difference in the mean relative growth rate over the experimental period between seedlings injected with 0 and 1000-3000 nematoda. However, in the case of seedlings without tree tonic, the relative growth rate of seedlings injected with 1000 or 3000 nematoda became down significantly more than those without nematode at two or three months after nematode injection. The facts suggest that the tree tonic significantly increases the vitality and biomass increment of black pine seedlings, and suppresses the negative effect of nematode invasion.