Functionalized fullerenes and carbon nanotubes were investigated as regards in vitro cytotoxicity and hemolytic properties. Pristine carbon nanotubes (CNTs) were first shortened to make them compatible with cellular dimensions before functional groups were appended to their surface either covalently (e.g. amine, alcohol, carboxylate) or non-covalently (adsorption and polymerization of different polyethylene glycol-based amphiphiles). C60 fullerenes were covalently functionalized with polyethylene glycols of various sizes. Cell viability was measured 24 h after exposure to the nanomaterials using MTT and LDH assays which were adapted to avoid nanomaterial interference. In vitro analysis of hemolytic properties was also performed to assess acute damage to red blood cells. While all the tested nanomaterials were found to reduce, to some extent, the cellular metabolic activity, two only affected the plasma membrane integrity, and none induced hemolysis.