The study of chromosome aberrations produced by the irradiation in vivo of men dates from 1960, and nearly all the work has been done on cultures of blood in which lymphocytes respond to phytohæmagglutinin by synthesising DNA and then dividing (Moorhead et al. 1960). This technique can now be used on a moderately extensive scale and it is possible to trace the pattern of events after a single exposure or repeated exposures to radiation involving either the whole or part of the body, whether the exposure be for therapeutic purposes, or from the explosion of a nuclear weapon, or accidentally received, or incurred for occupational reasons. We can now detect the effect of radiation in producing an increased number of lymphocytes in the peripheral blood with chromosome aberrations with quite low doses of radiation, for example an occupational dose of about 15 to 20 rads. accumulated over a few years or the doses received during fluoroscopic procedures in diagnostic radiology. For the future we anticipate that the development of automated techniques of chromosomes counting and analysis will permit quantitative work down to such levels and perhaps even lower.