Changes in endocytosis in Alzheimer's disease (AD) were originally identified more than 15 years ago but it was not known whether they were causative or an effect of the disease. More recent studies on the genetics of AD have identified several genes encoding endocytic proteins which are associated with a small, increased risk of developing AD, strongly suggesting that changes in endocytosis are indeed involved in the aetiology of the disease. One of these genes encodes for the protein phosphatidylinositol binding clathrin assembly protein (PICALM). The ‘Amyloid Hypothesis’ suggests that the build-up of amyloid-beta (Aβ) is a primary causal event in AD. The Aβ peptide is cleaved from amyloid precursor protein (APP) by the β- and γ- secretases, primarily after APP has undergone endocytosis and within the endosomal/lysosomal system. We examined whether decreasing the expression of PICALM affected the processing of APP into Aβ to further our understanding of the importance of the role of this protein in the pathophysiology of AD. Levels of PICALM were depleted by siRNA using oligofectamine for 48 hours in human brain-derived H4 neuroglioma cells that endogenously express APP. Cells were lysed and media collected to detect intra- and extracellular protein levels of endocytic-related proteins, APP and APP metabolites including Aβ. Levels of endocytosis were quantified using ALEXA 488-conjugated transferrin as a marker of clathrin-mediated endocytosis and quantified using flow cytometry. PICALM levels were depleted by more than 85% with siRNA. Levels of intracellular APP, intracellular β-CTF and secreted sAPP β (APP fragments produced by β-secretase cleavage) were significantly reduced suggesting an inhibition of the amyloidogenic pathway. Meanwhile levels of transferrin uptake were significantly reduced in PICALM siRNA-treated cells after 15 and 30 minutes suggesting an inhibition of endocytosis. These results suggest that in cells with endogenous levels of APP, representative of levels of APP in AD in man, PICALM can modulate the amyloidogenic processing of APP to Aβ, primarily via its effect on levels and rates of endocytosis.