We showed previously that tungsten carbide–cobalt (WC–Co) nanoparticles (NP) can be used as a nanoparticulate positive control in some in vitro mammalian genotoxicity assays. Here, we investigate the mechanisms of action involved in WC–Co NP genotoxicity in L5178Y mouse lymphoma cells and primary human lymphocytes, in vitro. Data from the micronucleus assay coupled with centromere staining and from the chromosome-aberration assay show the involvement of both clastogenic and aneugenic events. Experiments with the formamidopyrimidine DNA glycosylase (FPG)-modified comet assay showed a slight (non-significant) increase in FPG-sensitive sites in the L5178Y mouse lymphoma cells but not in the human lymphocytes. Electron paramagnetic resonance spin-trapping results showed the presence of hydroxyl radicals (radical dotOH) in WC–Co NP suspensions, with or without cells, but with time-dependent production in the presence of cells. However, a significant difference in radical dotOH production was observed between human lymphocytes from two different donors. Using H 2 O 2 , we showed that WC–Co NP can participate in Fenton-like reactions. Thus, radical dotOH might be produced either via intrinsic generation by WC–Co NP or through a Fenton-like reaction in the presence of cells.