Abstract
1. Arvin stimulated the reticulo-endothelial uptake of colloidal carbon in rabbits. The onset of effect after intravenous administration was immediate and the minimum stimulant dose by this route was very low-0.05 unit/kg (0.1 mug protein/kg).2. Carbon opsonized in vitro with serum taken 4 h after an intravenous injection of Arvin was cleared more rapidly than carbon treated with control serum. Serum collected 10 min after Arvin did not promote carbon uptake.3. Reticulo-endothelial block with Thorotrast or excess carbon did not prevent the stimulant effect of Arvin but prevented the stimulant effect of opsonization with 4 h serum.4. The reticulo-endothelial stimulant effect of Arvin appears to be unrelated to its coagulant action on fibrinogen. The mechanism may involve a direct action on phagocytic cells and an effect on colloidal particles via serum opsonins.