Abstract: A cross-linked, cellular elastomeric composition is made from a formulation which includes one or more natural and/or synthetic rubbers, a cross-linking agent, a monomeric metallic salt of an ethylenically-unsaturated carboxylic acid and a blowing agent. The metallic salt is preferably basic zinc methacrylate. The composition may be used to provide a core for a tennis-ball or a sponge for a table-tennis bat. The former use enables advantage to be taken of the properties hitherto only found in pressurized playballs, while the latter use leads to a marked increase in the speed of the ball when struck by the bat.