Peripheral blood lymphocytes from unirradiated control subjects and workers exposed within permitted limits to gamma-radiation, have been examined for the incidence of dicentric and acentric chromosome aberrations. The results are compared with a review of data published elsewhere. Background levels show inter-laboratory variation and possible reasons for this are discussed. By combining the present data with those from the literature the spontaneous incidence of dicentric aberrations in approx. 0.55 X 10(-3) and for acentrics is 3.7 X 10(-3). In occupationally exposed subjects a significantly higher incidence of aberrations was found. When allowance was made for the turnover of lymphocytes for the period over which each man had worked with radiation a linear dose--effect relationship was apparent. The incidence of dicentrics was 2.22 +/- 0.94 X 10(-4) rad-1 and for all unstable aberrations 8.24 +/- 2.8 X 10(-4) rad-1. These are in reasonable agreement with dose--response data obtained in vitro.