The transplantation of mobilized progenitor cells after high-dose chemotherapy shortens haemopoietic engraftment. CD34 cell subsets were examined in 20 consecutive mobilized progenitor cell collections obtained from patients with solid tumours that had not been previously treated. The analysis of CD34 cells was based on the expression of intracellular antigens, surface antigens including CD38, and cell size using multi-dimensional flow cytometry. We also correlated the numbers of stem cell subsets reinfused to haemopoietic recovery. The majority of CD34+ cells expressed CD13 and CD33. A significant proportion was cytoplasmic myeloperoxidase (cMPO) positive. CD34+ MPO+ cells increased significantly in late collections. MPO expression was related to cell size. Cells expressing CD13 also increased in late collections in parallel to CFU-GM count. Small subpopulations of CD34+ CD38+ were committed to B cells, T cells and erythroid cell lineages. A small population expressing the megakaryocytic antigen had a small size and were predominantly CD38-. A minor subpopulation expressed stem cells antigens. These were significantly higher in late collections (CD34+ Thy-1+ and CD34+ CD33-). After mobilization, patients received three cycles of intensive chemotherapy followed by reinfusion of mobilized progenitors (5.45 x 10(6)/kg CD34+ cells, range 3.4-11.88). The numbers of reinfused CD34 cells or the individual subsets did not influence recovery of leucocytes (9 d) or platelets (9 d). In conclusion, the numbers of stem cells and their subsets differed between collections and, in unpretreated patients receiving intensive chemotherapy, there was no delayed engraftment when sufficient numbers of stem cells were reinfused. The recovery period was short and not correlated to any stem cell subsets.