Continuous exposure to plastic pollutants may have serious consequences on human health. However, most toxicity assessments focus on non-environmentally relevant particles and rarely investigate long-term effects such as cancer induction. The present study assessed the carcinogenic potential of two secondary nanoplastics: polyethylene terephthalate (PET) particles generated from plastic bottles, and a biodegradable polylactic acid material, as respective examples of environmentally existing particles and new bioplastics. Pristine polystyrene nanoplastics were also included for comparison. A broad concentration range (6.25-200 mu g/mL) of each nanoplastic was tested in both the initiation and promotion conditions of the regulatory assessment-accepted in vitro Bhas 42 cell transformation assay. Parallel cultures allowed confirmation of the efficient cellular internalisation of the three nanoplastics. Cell growth was enhanced by polystyrene in the initiation assay, and by PET in both conditions. Moreover, the number of transformed foci was significantly increased only by the highest PET concentration in the promotion assay, which also showed dose-dependency, indicating that nano PET can act as a non-genotoxic tumour promotor. Together, these findings support the carcinogenic risk assessment of nanoplastics and raise concerns regarding whether real-life co-exposure of PET nanoplastics and other environmental pollutants may result in synergistic transformation capacities.