Graded X-ray doses up to 6.0 Gy were delivered to the rear 1/3 of body length (congruent to 1/3 body mass) of Syrian hamsters and 24 h later, peripheral blood lymphocytes were sampled, set up in culture together with bromodeoxyuridine and first post-stimulation mitotic cells were scored for chromosome-type aberrations. Very few aberrations were found at any dose and the response curve for centric exchanges ("dicentrics + centric rings") had no significant curvature or slope. Single irradiations of 3.0 Gy delivered to the whole body and sampled by identical protocol produced yields identical to, or slightly exceeding, those found for blood irradiated with the same dose in vitro. The same dose delivered to the front 2/3 or front 1/3 (head) or localised to the spleen area, and aberrations scored from blood or spleen-pulp lymphocytes, gave variable yields, but always higher than those obtained from the rear 1/3 irradiations. The results are consistent with dose-dependent interphase death of irradiated cells, coupled with a non-uniform distribution of lymphocytes within body tissues. It is estimated that some 80-90% of the lymphocytes are resident within the anterior 2/3. Detailed studies of the between-cell aberration distributions give evidence that positive selection against cells with high aberration frequencies has also occurred in these experiments.