The effects of twenty-three disaccharide esters of fatty acids on the phagocytic response of the reticuloendothelial system (RES) were examined by measuring the intravascular clearance of colloidal carbon 24 h after a single intraperitoneal injection into ddY mice. A marked increase in the phagocytic activity was demonstrated with the following samples : i) nine lots of industrially produced sucrose-tallowates containing the mono- and higher esters of stearic and palmitic acids (ca. 2-4 : 1) in various ratios ranging from 3 : 7 to 7 : 3, and ii) three laboratory-prepared samples, that is, sucrose-(di)- and -(poly)-stearates and trehalose-(di)-stearate. The phagocytic indexes shown by these agents at doses of 100 and 50 mg/kg, and, in some cases, even at 25 mg/kg were higher than, or comparable to, that obtained with 50 mg/kg of zymosan. Such a significant RES-stimulation could not be observed with other samples, i.e., a lot of commercial sucrose-tallowate consisting almost entirely of monoesters and a commercial sucrose-palmitate containing the mono-, di- and polyesters, ii) laboratory-prepared samples such as sucrose- and trehalose-(mono)-stearates, and -(mono)-myristates, maltose-(mono)-, -(di)-, and -(poly)-stearates and maltose-(mono)- and -(poly)-palmitates, and iii) sucrose and stearic acid in the free form.