1. The defence mechanisms of the body are predominantly functions of the reticulo-endothelial system (RES). Stimulation of the RES leads to increased body defence indicated by increased phagocytic activity, raised serum \g=g\-globulin and increased protection of experimental animals against virulent infections. The strongest RES stimulants appear to be oestrogens, natural and synthetic.2. In the mouse, 0\m=.\002 mg. diethylstilboestrol daily is sufficient to produce significant prolongation of survival time against infection.3. RES activity varies at different stages of the oestrous cycle and of pregnancy in both the rat and the mouse. The stages where the activity is greatest correspond with those in the human subject when the output of oestrogens is increased.4. The results suggest that oestrogen is the natural stimulant of body defence in both the male and female, the latter having high oestrogen levels when protection against infection is normally most needed. 5. RES stimulation does not rise in step with oestrogenicity, and oestrogenic compounds have two separate biological activities\p=m-\onewhich stimulates the RES to raise body defence, and one which acts on the reproductive organs; these two activities are apparently unrelated, although shared by the same molecule.

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