The synthesis, characterization, and toxicity of graphene oxide and reduced graphene oxide are reported. Prior to the cytocompatibility tests the stability of the suspensions in a wide range of concentrations (3.125-100μg/mL) of three different dispersants is studied. Polyethylene glycol (PEG), polyethylene glycol–polypropylene glycol–polyethylene glycol (Pluronic P123), and sodium deoxycholate (DOC) are investigated as the dispersants. The toxicity depends on the type of dispersant and concentration of the nanomaterials in the suspensions. Detailed analysis suggests that graphene oxide functionalized with PEG in the concentration range between 3125μg/mL and 25μg/mL exhibits the best biocompatibility with mice fibroblast cells (line L929).