A total of 258 aphereses were performed in 79 patients with nonmyeloid malignancies after mobilization of peripheral blood stem cells (PBSC) with recombinant human granulocyte colony-stimulating factor (rhG-CSF). Apheresis products were examined for viable mononuclear cell (VMC), CD34+ cell, and clonogenic cell contents. The number of progenitors in aphereses differs in subgroups of patients with different diagnoses. However, the number of CD34+ or clonogenic cells is dependent on age and amount of chemotherapy delivered to patients before collection rather than on the nature of the disease itself. In addition, the actual dose of rhG-CSF used to mobilize PBSC and the number of VMC in aphereses influenced the clonogenicity of CD34+ cells, although the daily dose of rhG-CSF seems to play little role on the number of clonogenic cells in each individual apheresis product. CD34+ cell and CFU-C (or CFU-GM) numbers are related parameters, and the relation can be described as linear. However, the linear relation varies in different patient groups, and most of the linearity is induced by the highest sets of values. We conclude that mobilization with low doses of rhG-CSF alone is feasible and that the probability of collecting a high number of peripheral blood progenitors is increased in young patients undergoing apheresis early in the course of the disease. Although the relationship between CD34+ cells and CFUs can be described as linear in well-defined situations, its relevance may be limited because it is not a universal finding.