The mean inactivation dose (D̄) is calculated for published in vitro survival curves obtained from cell lines of both normal and neoplastic human tissues. Cells belonging to different histological categories (melanomas, carcinomas, etc.) are shown to be characterized by distinct values of D̄ which are related to the clinical radiosensitivity of tumors from these categories. Compared to other ways of representing in vitro radiosensitivity, e.g., by the multitarget parameters D₀ and n, the parameter D̄ has several specific advantages: (i) D̄ is representative for the whole cell population rather than for a fraction of it; (ii) it minimizes the fluctuations of the survival curves of a given cell line investigated by different authors; (iii) there is low variability of D̄ within each histological category; (iv) significant differences in radiosensitivity between the categories emerge when using D̄. D̄ appears to be a useful concept for specifying intrinsic radiosensitivity of human cell lines.