An international seminar on the health effects of internally deposited radionuclides, with emphasis on radium and thorium, was held in Heidelberg, Germany, on 18-21 April 1994. The seminar was supported by the European Commission, the US Department of Energy, the German Federal Ministry of Environment and Reactor Safety and the German Cancer Research Centre. During the six years since the previous meeting on this important area of human radiobiology, new molecular biological techniques for investigating cancers had been developed and the seminar offered a timely opportunity to review the current state of knowledge on the induction of cancer in humans by internally deposited radionuclides. The topics covered were broadened to include not only radium and thorium, but to cover other radionuclides as well, such as polonium, plutonium, americium and strontium. The use of epidemiological data for radiation protection requirements is dependent on a knowledge of the accumulated dose to target cells. Therefore, since biophysical measurements, calculations and animal experiments are crucial for understanding the human data, more emphasis was given at the seminar to creating a bridge between epidemiology, molecular biology, cancer genetics and low dose effects. The proceedings of this seminar have now been published (442 pages) and include papers on the following subjects: - Internal dosimetry schemes, their physical, biological and epidemiological basis; - Dosimetry; - Biokinetics and analytics; - Epidemiology (radium, thorotrast, plutonium); - Animal studies in comparison with the human situation; - Mechanisms of internal radiation carcinogenesis; - Molecular biology investigations and genetic background.