Subcutaneous histiocytes (fixed ma crophages) are normal residents in the srtcutarecus loose connective tissue. Since the fact that histiocytes have high pliagccytotic activity and constitute a part of the reticuloendothelial system was established (Kiyono,1919; Aschoff,1924), extensive works concerning histiocytes have been performed (see the recent review by Pearsall and Weiser,1970). However, the ultrastructural investigations of histiocytes are rather scanty (Tanaka,1958; Kajikawa and Hirono,1960; Ross and Benditt,1962; Sato,1965; Kajikawa et al.,1970), and there have been practically no ultracytochemical studies of histiocytes as yet.Since the original reports of Graham and Karnovsky (1966a, b) who introduced the horseradish peroxidase as a tracer of exogenous protein in electron microscopy, quite an amount of works using exogenous peroxidase as a tracer have been published. Many of these are, however, the works on tracing the peroxidase injected intravascularly (Becker et al.,1967; Brightman,1967; Karnovsky,1967; Reese and Karnovsky,1967; Sommer and Johnson,1968; Becker and Almazon,1968; Bodenheimer and Brightman,1968; Graham and Kellermeyer,1968; Holtzman and Dominitz,1968; Hugon and Borgers,1968; Schneeberger-Keely and Karnovsky,1968; Clementi and Palade,1969; Graham et al.,1969Holtzman and Peterson,1969; Forssmann and Girardier,1970; Like,1970; Fahimi,1970; Jewett et al.,1971; Schneeberger and Karnovsky,1971). Some works tracing the peroxidase injected extravascularly or the peroxidase challenged in vitro have been published (Collins,1967; Friend and Farquhar,1967; Locke and Collins,1968; Cotran and Karnovsky,1968; Seder,1969; Zacks and Saito,1969; Watanabe et al.,1969; Holtzman et al.,1970; Cotran and Litt,1970; Peyman et al.,1970). However, no works have been published on the mode of exogenous peroxidase uptake injected subcutaneously. In the present study, the mode of exogenous peroxidase uptak e and the reactive changes of the lysosomal system accompanying with uptake are investigated in the mouse subcutaneous histiocytes using ultrastructural and ultracytochemical technics.