Aim of this work was to investigate whether a surface coating with poly(ethylene glycol) (PEG) makes halloysite clay nanotubes (HNTs) biocompatible up to a concentration level suitable for their effective employment as ultrasound contrast agents. Such a concentration level has been recently identified in a contrast dose of 1.50 mg/mL., which has been shown to be particularly suitable for automatic HNT detection within echographic images acquired through a clinically-available ultrasound device employing conventional diagnostic settings. The dose- and time-dependent cytotoxicity of uncoated and PEG-coated HNTs was evaluated in vitro by MTT cell viability assay carried out on two different human cell lines: SKOV-3 (human ovarian carcinoma cells) and MG-63 (human osteosarcoma cells). Cells were each time incubated for 6 to 72 h in the presence of HNTs at a concentration ranging from 0.25 to 1.5 mg/mL …