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If the depilated skin of white mice is rubbed with solutions of histamine or certain derivatives of bicyclo[0.3.5]decapentaene, simultaneous intravenous injection of a suspension of India ink leads to an intracellular accumulation of ink particles inside the endothelial cells of small vessels at the site rubbed. This phenomenon of phagocytosis is induced in cells which have normally no phagocytic capacity and can be inhibited by antihistaminics. Solutions of these derivatives are quickly absorbed and are thought to liberate sufficient quantities of histamine to induce phagocytic activity of the endothelial cells. Some derivatives of the bicyclo[0.3.5]decapentaene skeleton have been tested for similar action and a quantitative difference was observed. These observations confirm those of Jancsó who states that histamine stimulates the endothelial cells of small vessels to phagocytic function. The phenomena described seem to have a certain importance because if the hypothesis is correct, a high number of normally inactive cells may be placed at the disposal of the natural cellular defense mechanism of the organism by means of a physiological stimulus.

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A series of compounds are described which exhibit therapeutic activity in experimental tuberculosis, acting exclusively on the host. The active compounds (citronellyl acetate, d-limonen, linalyl acetate) induce a local India ink and trypan blue accumulation and also stimulate the macrophages in vitro and in vivo to an increased phagocytic activity. Sera of the animals treated with therapeutically active compounds stimulate the macrophages of normal animals to an increased phagocytosis. A correlation between chemical structure of the reticulo-endothelial system stimulating compounds and the above effects is discussed.