This study into the effect of polypropylene fibres on concrete strength accompanies research being carried out into the possible enhancement of fire resistance of high grade concrete from polypropylene fibres. It was found that the addition of polypropylene fibres to concrete causes a loss of cube compressive strength of about 2% per kg/m3 of fibres, up to a content of 6 kg/m3. This loss is related to the reduction in density due to the addition of polypropylene fibres. It may directly affect the design of structural elements.There is no significant loss of flexural strength nor cylinder splitting strength from the addition of polypropylene fibres. 16 pages. This publication is supplied as a photocopy .