In this supplement to both the American and the European protocols for clinical neutron dosimetry, new recommendations are given with respect to the basic physical parameters and experimental techniques employed. For neutron dosimetry, the air kerma or exposure calibration in a photon beam is the most suitable method for the calibration of tissue-equivalent ionization chambers until calibration in a standard neutron field becomes available. More recent data are recommended for the physical parameters required for the photon calibration as well as for the measurements in the neutron beam. Water is recommended as the reference phantom material due to its similarity in absorption and scattering properties to muscle. The resulting overall change in absorbed dose calculated according to this supplement, compared with the original protocols, will be smaller than about +/- 2% due to differences in the basic physical parameters. An additional change of several percent occurs at depth in a phantom as a result of the difference between water and the muscle-equivalent liquid formerly recommended as the reference phantom material.