Halloysite Clay nanotubes (HNTs) are naturally occurring nanotubes composed of double layered aluminosilicate minerals with a hollow tubular structure in the nanometer range. In recent years, HNTs have attracted specific research attention as a possible new material for various biological applications, including drug and gene delivery vehicles, ultrasound contrast agents, cancer and stem cells isolation. Therefore, assessment of HNT biocompatibility has gained importance to demonstrate its suitability for clinical purposes. In this study, HNTs were densely coated with poly(ethylene glycol) (PEG) and MTT measurements were performed on MCF-7 (breast cancer) and HeLa (cervical cancer) cells to quantify the biocompatibility of PEG-coated HNTs as a function of nanotube dosage and incubation time. PEG-coated HNTs resulted fully biocompatible for both cell lines at concentrations up to 0.1 mg/mL (>70% of cells survived after 72-h incubation), making them suitable candidates for nanomedicine applications at moderate levels of exposure.