Antibodies raised against the Sarkosyl-insoluble, major flagellar glycoprotein fraction, mastigonemes, were used to determine the source of flagellar surface glycoproteins and to define the general properties of flagellar surface assembly in Euglena. After suitable absorption, mastigoneme antiserum reacts with several specific mastigoneme glycoproteins but does not bind either to the other major flagellar glycoprotein, xyloglycorien, or to other Sarkosyl-soluble flagellar components . When Fab' fragments of this mastigoneme-specific antiserum were used in combination with a biotin-avidin secondary label, antigen was localized not only on the flagellum as previously described but also in the contiguous reservoir region . If deflagellated cells are reservoir pulse-labeled with Fab' antibody, this antibody appears subsequently on the newly regenerated flagellum. This chased antibody is uniformly distributed throughout the length of the flagellum and shows no preferred growth zone after visualization with either fluorescein or ferritin-conjugated secondary label. From these and tunicamycin inhibition experiments it is concluded that (a) a surface pool of at least some flagellar surface antigens is present in the reservoir membrane adjacent to the flagellum and that (b) the reservoir antigen pool is transferred to the flagellar surface during regeneration .Prominent among the surface features of the emergent flagellum of Euglena is the flagellar sheath consisting of nearly 30,000 precisely positioned mastigoneme filaments and their complex anchoring units (7), and a continuous 200-A membrane fuzzy layer (33) . These well-defined surface features are limited in their overall surface distribution and endow the flagellum with a distinct biochemical profile (7) not directly comparable with the rest of the Euglena cell surface (19) or to a number of other recently analyzed flagellar/ciliary membranes (1,4,5,6,10,11,16,20,25,27,28,31,36,40,41) . These specialized membrane and surface properties exhibited by Euglena and other flagella raise interesting questions with respect to the mechanisms of synthesis, the pathways of mobilization from sites of synthesis, and the timing of surface assembly relative to axonemal growth . Some of these questions can be addressed in Euglena because two major flagellar glycoproteins have been identified and isolated and their structural counterparts in the flagellum confirmed (33) .For example, neutral detergent extraction of Euglena flagella yields "xyloglycorien" which comprises a 200 A fuzzy surface 758 layer, whereas flagella solubilized with Sarkosyl leave behind an insoluble particulate fraction consisting of surface "mastigonemes ." While both of these glycoproteins share in common the presence of the pentose sugar xylose (7, 15), they can be readily distinguished by (a) their structure and association with the flagellar surface, (b) their molecular weights, and (c) their sensitivity to degradation by proteases (32) . Results from the use of antibody probes specific to xylogl...