The effect of storage for periods of up to two years in bovine serum at 37 degrees centigrade on the mechanical properties of Simplex and CMW bone cements was studied. No significant deterioration was detected either in static properties or in compression fatigue behavior. The data on compression fatigue suggest that working stresses in the cement be limited to less than 1.4 x 10-8 dynes/cm2 to ensure long-term viability of any design or implantation technique.