Taking advantage of size differences between neoblasts and differentiated cells of the flatworm, serial filtration and density gradient centrifugation were used to prepare fractions highly enriched in either neoblasts or differentiated cells.these fractions were then introduced into flatworms that had previously been irradiated with a lethal dose of X-rays. Following irradiation,flatworms normally lose their ability to regenerate and they die within 3-5 weeks. However, when irradiated flatwarms were injected with the fration enriched in neoblasts, regenerative abilities and long-term survival could be restored. In contrast, injection of the differentiated cell fraction into irradiated planaria did not rescue the effects of irradiation.