This paper is a review of the work of the Cytogenetics Laboratory of the National Radiological Protection Board during the last 4 yr. A brief account of the culture and scoring techniques is given. Calibration curves have been obtained for fission neutrons, X-rays and60Co gamma rays. These have been used to estimate equivalent whole body dose from blood samples taken from 67 cases of known or suspected over-exposure investigated in the laboratory. Studies have been carried out on groups of radiation workers and radiotherapy patients. Data from some patients treated with colloidal198Au provide an example of how the study of the distribution of dicentrics among cells can give useful information about partial body irradiation.