Methods of potentiating chemotherapy or radiotherapy are disclosed. The methods comprise administering to a subject in need of chemotherapeutic or radiotherapeutic treatment an effective amount of a composition comprising biocompatible nanoparticles, particularly gold nanoparticles, under conditions in which the nanoparticles alter one or more cell regulatory mechanisms in cells in which the nanoparticles are localised or other cells. Then one or more doses of a chemotherapeutic or radiotherapeutic treatment are administered to the subject either concurrently with or after the nanoparticles have altered the one or more cell regulatory mechanisms in the cells in which the nanoparticles are localised or other cells. Also disclosed are methods of enhancing the effects of chemotherapy or radiotherapy on a cell population, methods of increasing the amount of strand breaks in DNA in a cell, and methods of inducing cancer cell death.