Objective: To determine the mechanical-support-equivalency of three materials when used as bases below a composite restoration. Base materials must develop adequate stiffness to support the restoration during masticatory loading. The stiffness of a new calcium silicate dentin substitute (B, Biodentine, Septodont) and two commercially successful glass ionomer bases (F, Fuji IX, GC America; V, Vitremer, 3M ESPE) were compared at 4 curing times when an indenting load was applied to a simulated Class I composite restoration.