[en] This introductory talk on immunological indicators will be restricted to own experiments on the effects of ionizing radiation, used as a therapeutic tool, on the central cell of the immunological apparatus - lymphocyte. An account will be given of sequels observed in breast cancer patients treated by radiation as well as of effects caused by in vitro irradiation of lymphocytes. Two different approaches were used to study changes which take place in the immunological apparatus following in vivo or in vitro radiation. The composition of different lymphocyte subpopulations was examined by means of phenotypic markers and various in vitro functions were measured as determined on a cell for cell basis. Furthermore, it has been demonstrated that T and B lymphocytes interact and that T lymphocytes can be further divided into several functional subpopulations. (orig./MG)