Bloodstream (BSF) and culture forms (CF) of Trypanosoma lewisi were specifically agglutinated with the plant lectins concanavalin A (Con A), soybean agglutinin (SBA), wheat germ agglutinin (WGA), and fucose-binding protein (FBP). Lectin-mediated cell agglutination was inhibited, and reversed in the presence of specific lectin-binding saccharides. Cells were agglutinated randomly with all lectins suggesting a uniform distribution in the trypanosome cell surface of the lectin-binding saccharide ligands. The BSF and CF were not agglutinated with phytohaemagglutinin-M, phytohaemagglutinin-P, or influenza virions. Living trypsinized BSF, which lacked a surface coat, gave agglutination results with the lectins identical to those obtained with living intact BSF. Glutaraldehyde- or formalin-fixed intact and trypsinized BSF gave results similar to those obtained with living cells and SBA, WGA, and FBP. However, intact, fixed BSF gave much lower agglutination levels with Con A than trypsinized-fixed, living intact, or living trypsinized BSF cells. Intact and trypsinized living and fixed CF gave identical agglutination results with each of the lectins. Living and fixed cells treated extensively with the glycoside hydrolases alpha-amylase, dextranase, and neuraminidase gave results with the lectins identical to those obtained with untreated cells. Con A bound at the cell surface was visualized with an iron-dextran (Fe-Dex) conjugate. Dense iron marker particles were distributed randomly in the intact BSF surface coat. The Con A-bound Fe-Dex marker was present on the pellicular and flagellar membrane outer lamina of trypsinized BSF and intact CF cells. Horseradish peroxidase (HRPO)-diaminobenzidine (DAB) coupled reactions also were used to visualize surface-bound Con A. Dense Con A-HRPO-DAB deposits were present uniformly in the BSF surface coat, and on the membranes of trypsinized BSF and intact CF trypanosomes. SBA and WGA were conjugated to HRPO and these used in DAB-coupled reactions at the ultrastructure level. Results obtained with the HRPO-conjugated lectins were similar in surface localization and distribution to those obtained with the Con A-HRPO-DAB preparations. Treatment of BSF and CF with the several glycoside hydrolases produced no apparent enhanced or reduced reactivity for the lectins in any of the fine-structure cytochemistry experiments. The cumulative results indicate that ligands similar or identical to alpha-D-mannose, N-acetylgalactosamine, and N-acetylglucosamine, and alpha-L-fucose are constituents in the extracellular surface coat matrix of T. lewisi BSF. Similar conclusions also pertain to the pellicular and flagellar membrane ligands of the BSF and CF cells. Moreover, results obtained with the glycoside hydrolases and influenza virions suggest that the T. lewisi cell surface ligands are not associated directly with repetitively bonded alpha-I,4- and alpha-I,6-D-glucans or sialic acid moieties.