A microdosimetric model was used to analyze the results of experimental studies on cells of two lymphoid cell lines (T2 and Ada) irradiated with (213)Bi-radiolabeled antibodies. These antibodies targeted MHC/peptide complexes. The density of target antigen could be modulated by varying the concentration of the peptide loaded onto the cells. This offered the possibility of changing the ratio of specific (from cell-bound antibody) to non-specific (from antibody present in the supernatant) irradiation. For both cell lines, survival plotted as a function of the mean absorbed dose was a decreasing exponential. For the T2 cells, the microdosimetric sensitivity calculated for the whole cell was equal whether the irradiation was non-specific (z(0) = 0.12 +/- 0.02 Gy) or specific (z(0) = 0.12 +/- 0.09 Gy). Similar results were obtained for Ada cells. These results constitute a biological validation of the microdosimetric model. For both cells, the measured cell mortality was greater than the percentage of hit cells calculated with the model at low mean absorbed doses. This observation thus suggests bystander effects. It poses the question of the relevance of the mean absorbed dose to the cell nuclei. A new concept in cellular dosimetry taking into account cytoplasm or membrane irradiation and bystander modeling appears to be needed.