The mechanical properties of a number of commercially available bone cements have been investigated. Tests were carried out on specimens in compression, in bending and in tension. Using the compression test as a standard, the effects of the following variables were studied: the addition of antibiotics, strain rate, environmental temperature, and age. It was concluded that age, temperature and rate of straining have a marked effect on the strength of the cement, while the addition of small quantities of antibiotics only marginally weakens the cement.