The endocytic pathway of Tritrichomonas foetus, a parasitic protozoan of cattle, was studied using (a) vital dyes, such as Lucifer yellow, neutral red and acridine orange, (b) cationized ferritin, (c) gold-labeled lactoferrin and lectins: HPA, UEA, PNA and LPA, and (d) DAMP (3-(2,4-dinitroanilino) 3' amino-N-methyldipropylamine). Light and confocal laser microscopy as well as transmission electron microscopy were used in this study. Assays were monitored by fluorescence and electron microscopy after exposing the parasites to different conditions. Cells that were incubated at 15 degrees C or 20 degrees C with gold-labeled lactoferrin and processed for electron microscopy show that of 15 degrees C this ligand is found only in an early endosomal compartment and at 20 degrees C it is found in late endosomes but not in lysosomes. Immunocytochemical data from cryosections using DAMP as a pH probe show that T. foetus has acidic compartments, with a pH range of 5.2 to 6.6, with variable morphology, localization and size. Lectin-binding sites and anionic sites were also internalized and appear to be associated with membranes lining the vacuoles. Images of patching and shedding of these sites were also observed when HPA and UEA were used.