The internalization of host macromolecules to the vesicular fluid of T. crassiceps cysticerci was studied in vitro. Uptake of purified class G immunoglobulin was not significantly affected by the specificity of its antigen-recognition site and bovine serum albumin was internalized at a similar rate. Internalization was inhibited at low temperature, being optimal at 37 degrees C and saturation was accomplished only at a protein concentration in the culture medium over 12 mg/ml which is close to the physiological concentration of serum proteins in the host. Morphological studies using markers for adsorptive endocytosis allowed visualization of endocytic vesicles and tracking of their movement across the bladder wall tissue. Degradation of internalized proteins was observed at longer times of incubation, suggesting that proteins are later processed and that degraded host macromolecules can be nutrients for cysticerci. Quantification of this capability of internalization suggests that it might play a role in the in vivo removal of potentially damaging host macromolecules, such as antibodies or complement factors, from the host-parasite interface.