The protein binding in the kidney was studied in vivo by cationic ferritin microinfusion into rabbit proximal tubules. After ferritin infusion, the protein was located in the small endocytic vesicles, large endocytic vacuoles, dense apical tubules and lysosomes, initially it was in the apical part of the cells, then progressively with time it was transported to the basal part of the cells. In addition, feritin was seen in small tubulo-vesicular structures in the apical cytoplasm. These structures with a diameter of 0.03-0.05a’m were oftcn found around ferritin-loaded vacuoles and occasionally it connected to these vacuoles. Furthermore, small clusters of ferritin particles were found in the basolatcral intercellular spaces, after tubular microinfusion of ferritin 15 min later only small amounts were observed but the number of cluster increased with time after ferritin infusion. The study demonstrates the transcytosis of protein in rabbit proximal tubules, and describes a novel ferritin containing tubulo-vesicular structures.