Abstract: A conventional mattress construction which includes a series of interconnected BONNEL type coil springs together with top and bottom covering layers of fabric and padding is modified by the insertion into some of the coils of a circular cylindrical core member formed from foam which has a length greater than the length of the spring so that ends of the core member project outwardly beyond the end of the springs into engagement with the padding on the top and bottom surfaces of the mattress. The core member has a diameter which is a friction fit with the inside diameter of the intermediate turns of the spring. The core members are provided preferably in the center third of the mattress to increase stiffness in that area and to reduce vibrations being transmitted from one occupant to a second occupant.