The aim of this study was to assess the toxicological risks arising from the coexistence of polyethylene glycol coated single-walled carbon nanotubes (SWCNTs-PEG) and a known environmental contaminant: 4-nonylphenol (NP). To this end, in vitro toxicity assays involving the exposure of 3T3-L1 cells (mouse embryonic fibroblasts) to SWCNTs-PEG alone or in combination with NP for 24 or 48 h were performed. Experimental treatments were conducted in both presence (10%) and absence of serum in order to evaluate its influence on the toxicity of SWCNTs-PEG. Although the results provided no unambiguous evidences of synergistic toxicity between SWCNTs-PEG and NP, some specific treatments with mixtures (SWCNTs-PEG + NP) resulted in an unexpected combined toxicity in relation to the individual treatments. Only in those cases the interaction between SWCNTs-PEG and NP could have a synergistic effect on the resulting toxicity. The addition of 10% serum increased the stability of SWCNTs-PEG in the culture medium possibly by steric repulsions and reduced the toxicity of nanoparticles as a result. Overall, the serum had a "protective effect" on cells against all treatments: SWCNTs-PEG, NP or their mixtures (SWCNTs-PEG +NP). Raman spectroscopy allowed the intracellular distribution of SWCNTs-PEG to be elucidated. (C) 2014 Elsevier B.V. All rights reserved.