Adherence of granulocytes to the endothelium of small veins in different microvascular beds was quantified by visual counting of the adherent cells. Increased adherence of granulocytes induced by the stress of preparation or by ultraviolet light irradiation of the microvasculature was greatly reduced by i.v. administration of sialic acid. In the rabbit ear chamber laser irradiation induced increased granulocyte stickiness which was similarly prevented by intra-arterial infusion of sialic acid. In normal rats, sialic acid treatment induced granulocytosis and the effect was more striking on Busulphan-induced leucopenia in rats. Glucuronic acid applied under identical conditions had no effect on granulocyte adhesiveness in vivo or on the peripheral blood granulocyte count. According to these findings exogenous sialic acid greatly reduces the effect different pathogenic stimuli have of inducing adherence of granulocytes to blood vessel walls and induces granulocytosis by mobilizing cells from the "marginating" and reserve pools.