Endothelial cells have the natural property of releasing soluble factors into the fluid surrounding them, said factors altering the behavior of immune cells. In cell systems containing at least endothelial cells, the release of this type of factor, which promotes the proliferation of resting and weakly activated lymphocytes and at the same time inhibits the proliferation of highly activated lymphocytes and transformed lymphoblasts without impairing their other vital functions, is induced by administration of pentacyclic oxindole alkaloids but inhibited by the concurrent administration of tetracyclic oxindole alkaloids.