The 'fluence method' and the 'twin-detector method' have been employed to determine the relative neutron dose response, k_U, of several argon-filled magnesium ionisation chambers for neutron energies of 5.0, 7.9, 14.8 and 19.0 MeV. Differences in the value of k_U were observed from chamber to chamber although they were of a common design. The k_T value of an A-150 plastic tissue-equivalent ionisation chamber was also measured at these energies.