The influence of the flame retardant agents, antimony trioxide and a chlorinated hydrocarbon, on the mechanical and flammability properties of high density polyethylene has been studied. Flammability was assessed by means of the limiting oxygen index test, whilst mechanical properties were measured in the tensile mode on compression moulded samples. An optimum in terms of flame resistance was found at a Sb:Cl mole ratio of 1:3 which tends to confirm that the actual flame retardant is the volatile antimony trichloride. The modulus, yield and drawing behaviour, and ultimate properties of the unoriented samples did not show significant change (< 10%) until the combined level of additive exceeded 25% by wt. Above this level it was found that the samples could not be oriented and that the elongation to break decreased markedly. Alumina trihydrate was studied as an alternative flame retardant but was found to be unsuitable for use in HDPE, since to obtain an adequate level of fire retardancy a high concentration (40%) of additive was required, which resulted in a significant deterioration in the mechanical properties.