[en] Multiple-beam fringes of the Fizeau type in transmission and on reflection were utilised for the studies of the optical properties of γ-irradiated Dralon fibres. Irradiation was carried out over a period of 24-1948 h. The dose rate was maintained at 14.7 +- 0.2 rad s^-1. Empirical formulae were suggested to represent the variation of refractive indices and birefringence with the dose. Variation of the mean refractive index with the wavelength of light has been studied, and the constants A and B of Cauchy's dispersion formula were calculated. This multiple-beam interferometric method was also applied to determine the thermal coefficient of refractive index of the fibres. Micro-interferograms are given for illustration. (author)