The in vivo translocation of thyroxine-binding blood serum prealbumin (TBPA) was 
studied. It was found that the TBPA-hormone complex penetrates-through the 
plasma membrane into the cytoplasm of target cells. Electron microscopic 
autoradiography revealed that blood serum TBPA is localized in ribosomes of 
target cells as well as in mitochondria, lipid droplets and Golgi complex. 
Negligible amounts of the translocated TBPA is localized in lysosomes of the 
cells insensitive to thyroid hormones (spleen macrophages). Study of T4- and 
T3-binding proteins from rat liver cytoplasm demonstrated that one of them has 
the antigenic determinants common with those of TBPA. It was shown 
autoimmunoradiographically that the structure of TBPA is not altered during its 
translocation.