[en] The in-situ irradiation-induced attenuation of a number of commercially available multimode optical fibres has been studied as a function of temperature and dose-rate using ⁶⁰Co gamma-radiation. Silica fibres doped with germanium and phosphorus exhibited the lowest radiation sensitivity at doses < 10² Gy (10⁴ rads) and plastic clad silica (PCS) fibres at doses greater than this, up to 10⁵ Gy (10⁷ rads). The differences in radiation response for temperatures in the range 0⁰C to 100⁰C and dose-rates of 10²-10³ Gy h^-1 were small. (author)