We have studied the time course changes in committed granulocytic progenitor cells (CFC) in 54 pediatric patients treated by chemotherapy for a malignant non-Hodgkin lymphoma. This study has revealed the following points: 1) In the absence of patent bone marrow invasion, the bone marrow CFC concentration is, before treatment, less than normal; 2) Bone marrow invasion coincides with a large decline in bone marrow CFC concentration; 3) The aplasia induced by the initial chemotherapy is mediated by a quasi-complete destruction of CFC; 4) After regeneration from the initial therapeutic aplastic state the CFC exhibited a quite variable behavior from one patient to another. This phenomenon is probably linked to the variable recuperation time for each patient; 5) Following the conclusion of treatment, the patients, although having a normal peripheral leukocyte count, appeared to maintain a major aftereffect characterized by a clear and prolonged reduction in the concentration and total number of CFC.