Abstract: An opacifier substance is added to a plastic material which is naturally transparent to laser radiation. The opacifier substance is added in powder form as a concentration of 2% to 6% by weight of the plastic material. The opacifier substance is preferably antimony oxide and the resulting plastic material is capable of being permanently marked by a laser beam and is suitable for providing animal ear tags. These tags may be marked with an automatically readable bar code and this code continues to be automatically readable over a period of several years use on the animal.