The method of protecting a polymeric plastic from attack by bacteria and fungi includes providing bioactive glass particles having a mean particles size preferably less than 2 .mu.m as an additive incorporated into the polymeric plastic. The bioactive glass particles contain 40 to 60% by weight of SiO.sub.2, 10 to 30% by weight of CaO, 10 to 35% by weight of Na.sub.2O, 2 to 8 % by weight of P.sub.2O.sub.5, 0 to 25% by weight of CaF.sub.2, 0 to 10% by weight of B.sub.2O.sub.3, 0 to 8% by weight of K.sub.2O and/or 0 to 5% by weight of MgO. The additive is included in amounts of up to 25% by weight in the polymeric plastic.