Induction of structural chromosome aberrations was studied in human peripheral blood lymphocytes irradiated in the unstimulated state with ultrasoft X-rays. Aluminium K X-rays (1.49 ke V) and carbon K X-rays (0.28 ke V) were used. The frequencies of dicentric aberrations and of excess acentric fragments were found to increase approximately linearly with absorbed dose of both radiations. Carbon X-rays were more effective than aluminium X-rays. The corresponding linear yield coefficients for dicentrics are (3.7 X 10(-1)) Gy-1 and (2.2 X 10(-1)) Gy-1 respectively. AT low doses both these radiations are more effective than hard X-rays in producing dicentric aberrations, but at high doses aluminium X-rays are less effective than hard X-rays because of the large positive curvature of the hard X-ray dose-response. The most significant observation of these experiments is that electrons of less than 280 eV, produced by carbon X-rays, are efficient in producing dicentric aberrations. This implies that single local energy events of about 14 ionizations with in less than 7 nm are able to produce exchange aberrations.