In this study we have investigated by means of electron microscopy the uptake of the fragment crystallisable gamma receptor (FcãR) and its ligands (aggregated immunoglobulin G (AgIgG)) in J774 cells, a mouse macrophage cell line, and in rat liver endothelial cells (LEC). The results show that the uptake of ligands follows the classical pathway of receptor-mediated endocytosis: internalization in clathrin-coated pits and coated vesicles, delivery to endosomes, and finally to lysosomes. We also demonstrate that the binding of AgIgG to FcRs leads to the internalization and degradation of both ligand and receptor. The fact that both receptor and ligand were degraded suggests that interaction of FcR with the ligand removes the receptor from the constitutive recycling pathway and causes its transport to lysosomes. The degradation of IgG as well as its receptors begins after about 20-30 min of continuous endocytosis suggesting that the proteolysis occurs in multi-vesicular bodies (MVB)/late endosomes, which contains functionally active enzymes. However the complete degradation of IgG and its receptor is thought to occur in mature lysosomes.