[en] A preliminary study was performed to estimate radiation dose in 10 individuals who had entered Hiroshima city one day after the explosion and worked around the hypocenter area for a week. The numbers of cells with chromosome aberrations differed in each subject, ranging from one to 18 (median: 11 abnormalities) in 500 metaphases of lymphocytes cultured for 48 hours with phytohemagglutinin. Five individuals had more than 1.6 percent stable type aberrations and were estimated to have been exposed to 6-13 rad. The other five individuals and the controls were estimated to be less than two rad. (author)