Four children with chronic myeloproliferative disorders (three with Philadelphia [Ph1] chromosome-positive chronic myelogenous leukemia [CML] were studied with soft agar culture at diagnosis (before therapy) in an attempt to define abnormalities in granulopoiesis. The three patients with CML had elevated peripheral blood golony-forming cells (CFCs) and/or normal or decreased bone marrow CFCs (in those studied). Colony-stimulating activity (CSA) was markedly decreased or absent at diagnosis in all three. Maturation of myeloid cells eithin the colonies in agar was normal, indicating that no block in myeloid maturation was present. These findings are in general agreement with results previously reported in untreated adults with Ph1 chromosome-positive CML and further define the similarity with the adult form of the disease. One Ph1 chromosome-negative patient with a clinically similar chronic myeloproliferative disorder was studied and had similarly elevated peripheral blood CFCs. She had normal CSA with a similarly high WBC count. This finding was unexpected and suggests that, unlike the patients with CML, her monocytes were capable of elaboration CSA. This difference might prove helpful in the classification of this type of disorder in cases where the Ph1 chromosome abnormality is not present.