In this volume are collected 30 papers, 9 round table discus sions and 11 communications presented at the ASI Course on "The use of human cells for the evaluation of risk from physical and chemical agents," sponsored by NATO and organized by ENEA. The aim of the Course was to present different scientific ap proaches and technical advices in order to get dose-effect relation ships which are the basis for risk evaluation. The scientific back ground which is behind this approach was extensively discussed. Emphasis has been given to the use of human cells or human data in order to attempt to have a correct and realistic evaluation of the damage in humans. There are many criticisms on the use of animal data for human risk evaluation because of differences between species and between strains within the same species: differences in metabolism, activa tion processes and DNA repair ability makes uncertain the extrapola tion of animal data to humans. Also data obtained using specific strains or highly inbred strains in order to reduce the variance are not applicable due to the heterogeneity of the human population connected with individual responses. In this respect only the use of human cells enable us to detect the individual variability and to identify sensitive subpopu lations that would be at greater risk. My appreciation to Pieranita and Alberto Castellani for the as sistance during the meeting and to Giuseppe Biondi for his help in some of the editorial work."

This book is a collection of some of the papers presented at the EMBO Lecture Course on "Lymphocyte stimulation: differential sensitivity to radiation; biochemical and immunological properties." The Course was organized with the aim of fostering interactions between photoradiobiologists and immunologists interested in the problem of DNA damage and repair studied at the lymphocyte level. The papers presented in this book are mainly centered on the problem of radiation sensitivity of lymphocytes in relation to DNA repair phenomena. The radiation biology of human lymphocytes is dominated by two phenomena: (a) high radiosensitivity of lymphocytes which die in interphase (b) PHA-induced relative radioresistance of those cells which, after stimulation, escape the interphase death and eventually die in mitosis. These phenomena constitute a good system to study some of the factors which control the response of human cells to irradiation. In addition it is possible to correlate the development of the relative radioresistance in PHA-stimulated lymphocytes with the biochemical changes connected with the transformation processes. The papers presented in this book constitute a real contribution to the scientific knowledge in this field of research and suggest that lymphocytes could be a very interesting test material useful for measuring the DNA repair capability of human cells to furnish an indication of individual radiosensitivity in man.