Herpetomonas samuelpessoai agglutinates when incubated with concanavalin A (Con A). Agglutination involved binding of Con A to specific receptors as it was inhibited by alpha-methyl-D-mannoside. The pattern and the intensity of the agglutination as well as the localization of Con A-binding sites were influenced by the composition of the growth medium, growth temperature and time of incubation. Con A-binding sites were detected by using the Con A-horseradish peroxidase-diaminobenzidine technique.