Abstract Nanosized zeolite particles are important materials for many applications in the field of nanotechnology. The possible adverse effects of these nanomaterials on human health have been scarcely investigated and remain largely unknown. This study reports the synthesis of nanozeolites Y and A with particle sizes of 25-100 nm and adequate colloidal stability for in vitro cytotoxicity experiments. The cytotoxic response of macrophages, epithelial and endothelial cells to these nanocrystals was assessed by determining mitochondrial activity (MTT assay) and cell membrane integrity (LDH leakage assay). After 24 h of exposure, no significant cytotoxic activity was detected for nanozeolite doses up to 500 μg/ml. The addition of fetal calf serum to the cell culture medium during exposure did not significantly change this low response. The nanozeolites showed low toxicity compared with monodisperse amorphous silica nanoparticles of similar size (60 nm). These results may contribute to the application of safe nanozeolites for purposes such as medical imaging, sensing materials, low-k films and molecular separation processes.