Human fibroblasts are a suitable model to investigate early and late radiation-induced damages because they are involved in most medical irradiation. Early damages of healthy tissue in the entrance channel of the beam and around the tumor are estimated on the basis of cell inactivation measurements (X-ray 250kV, 3.2 Gy/min.; carbon ions at 199 MeV/u, LET 16.2 keV/micrometer; carbon ions at 11MeV/u, LET 153.5 keV/micrometer). Late effects have been assessed by measuring premature differentiation and increased collagen secretion of fibroblasts. In the organism premature differentiation and increased collagen secretion are part of a fibrotic reaction due to a response of tissue cells (among fibroblasts) to primary parenchymal damage. 10 days after irradiation fibroblasts show a dose--and LET--dependent increased collagen secretion correlating with premature terminal differentiation.