With a view to the application of fast neutrons in medicine and biology the G-value for the standard (Fricke) ferrous sulphate solution was determined for 3 and 14 MeV neutrons by comparison with the ionometric method. Knowledge of the G-value enables us to use for dosimetry the chemical technique which is of advantage wherever the irradiated object does not have a simple geometry, as in the case in many biological and medical experiments.