1. Intravenous injection into dogs of rabbit anti-dog-spleen serum in doses of 0.04 cc./10 Kg. results in a significant increase in the mononuclear cells of the peripheral blood.
2. The rise in circulating mononuclears occurs promptly, with 24-48 hours, and is sustained, generally for 20 days.
3. A large dose of the same serum, 5.4 cc./10 Kg., exerted an opposite effect, poducing a significant decrease in circulating mononuclears.
4. The possible significance of these findings is discussed.