The availability of cultured lymphocyte preparations from radiation workers with internal deposits of plutonium provided the opportunity to examine whether irradiation of bone marrow cells had induced a transmissible genomic instability manifesting as an increase in de novo chromosome aberrations in descendant cells in the peripheral blood. The men were originally classified as having more than 20% of the maximum permissible body burdens of plutonium, and recent red bone marrow dose calculations provided individual cumulative estimates at the time of sampling ranging up to 1.8 Sv. The initial sampling occurred approximately 10 years after the main major intake, and samples were subsequently taken during three further periods over the following 20 years. Control samples were available from three of the four sampling times. Chromosome analysis of solid Giemsa-stained material revealed no significant differences either in comparisons between the total group of plutonium workers and controls for comparable periods or when the comparisons were restricted to a group of plutonium workers with initial red bone marrow plutonium doses greater than 0.25 Sv. However, the frequencies of cells containing chromatid exchanges, chromatid breaks, and chromosome and chromatid gaps decreased significantly over the study period for both the plutonium workers as a whole and the controls, and a similar fluctuating pattern was seen when sequential samples from groups of the same individuals were examined. Cells with dicentrics, centric rings and excess acentric fragments remained at similar frequencies throughout the study period. There was therefore no evidence from the study of blood lymphocytes for the induction of persistent transmissible genomic instability in the bone marrow of radiation workers with internal deposits of plutonium. The work has, however, confirmed the need for appropriate controls when conducting studies of cytogenetic end points of instability.