Renal uptake and degradation of cytochrome c and lysozyme were investigated, using preparations that were labelled by means of covalent coupling of either protein to iodinated tyramine-cellobiose. Following proteolytic digestion, the label remains 'trapped' within intracellular organelles. Within 15 min after intravenous injection, 43% of the [125I]tyramine-cellobiose-cytochrome c and 29% of the [131I]tyramine-cellobiose-lysozyme were recovered in the kidneys. Isopycnic sucrose-gradient fractionation indicates that the two proteins initially exhibit closely similar intracellular distributions, being associated with vesicles of an equilibrium density slightly lower than that of plasma membranes. However, within 5 min after injection, the two proteins exhibit distinctly different distribution profiles. The [125I]tyramine-cellobiose-cytochrome c is localized predominantly in the lysosomal fraction of the gradient. The [131I]tyramine-cellobiose-lysozyme is also translocated to the lysosomal fraction, but at a much lower rate. For both proteins, the rates of intracellular degradation correlate with their rates of translocation. The observed difference in their kinetics of intracellular movement suggests that the two proteins are translocated at different rates into transport vesicles.