Graphene-based nanomaterials are expected to have a profound impact on a broad range of applications. However, studies devoted to investigating the putative adverse health effects of these nanomaterials are hugely underrepresented in the current scientific literature. We have investigated the in vitro short-term cellular toxicity associated with graphene derivatives (GD): graphene oxide and reduced graphene oxide. This study focused on the toxicity of GD on two cell types.(i.e., epithelial cells and macrophages) found in the luminal aspect of the respiratory system, where the initial exposure to these materials is most prominent. Graphene oxide exhibited a mild cytotoxic action in comparison to carbon nanotubes on epithelial cells and macrophages. The interaction of the nanomaterial with the cell surface generated reactive oxygen species during the initial phase of the exposure and transmission electron microscopy studies showed that graphene oxide flakes of different sizes are taken up by cells via an endocy tic pathway, both in epithelial cells and macrophages. (C) 2013 Elsevier Ltd. All rights reserved.