The goal of this work was to appraise the after-effects of radiotherapy and associated radio- and chemotherapy on hematosarcomas by studying the granulocytic progenitor cells (CFUc). The bone marrow of 14 subjects was cultured in methylcellulose 1--6.5 years after completion of radiotherapy. The results were recorded as the number of CFUc per 10(5) nucleated medullary cells plated and per 10(5) metamyelocytes plated, and the total number of medullary CFUc was calculated. These different modes of expression are discussed. The post-radiotherapeutic effects appeared to be closely correlated with the volum irradiated. Chemotherapy seemed in several cases to accentuate the effects. Nevertheless, the organism can operate a powerful compensatory mechanism by means of increased mitosis, and in addition, certain subjects are able to maintain a normal number of CFUc after an aggressive treatment. It was possible in certain cases to study the medullary regeneration of the irradiated zone by culture of the CFUc and by the 59Fe external counting. Its extent after radiotherapy seemed for both the granulocytic and the red cell line to be correlated with the volume irradiated. Subject to the study of new cases, regeneration appears to be very reduced when chemotherapy is associated with radiotherapy.