Cell lines were exposed either to mixtures of gold and iron oxide nanoparticles, or to a hybrid nanoparticle with gold and iron oxide domain. In the case of simultaneous exposure to gold and iron oxide nanoparticles, enhanced toxicity as compared to the exposure to only one type of nanoparticles was observed. An indication was found that, at equivalent concentrations, the hybrid nanoparticles may slightly reduce cell viability more strongly than mixtures of both nanoparticle types. The results suggest that composite nanomaterials, in which different materials are present in particle form, need to be analyzed carefully, as not only the concentration of the respective materials but also their arrangement may influence their toxicity.