The use of peripheral blood progenitor cells (PBPC) to reconstitute hematopoiesis after high-dose chemoradiotherapy is now commonplace in the treatment of malignancies. Attempts to characterize these cells have concentrated primarily on their phenotype and their content of clonogenic colony-forming cells (CFC). We have used a plastic-adherent delta (P delta) assay system to evaluate the quantity and quality of more primitive cells in addition to the conventional measurements of CFC and CD34-positive cells. The leukapheresis products from 20 patients mobilized using cyclophosphamide (Cy) and granulocyte colony-stimulating factor (G-CSF) were examined for progenitor cell content. The mean number of mononuclear cells (MNC), colony-forming units-granulocyte/macrophage (CFU-GM), and CD34-positive cells from two leukaphereses per patients were 7.9 x 10(8)/kg, 47.3 x 10(4)/kg, and 10.5 x 10(6)/kg, respectively. The mean number of P delta progenitors was 9.3 x 10(4)/kg. Limiting dilution analyses showed the frequency of P delta progenitors in PBPC to be between 1 and 5.3 per 10(5) MNC and that each P delta progenitor has the proliferative capability to generate an overall mean of 4.5 CFU-GM. Of the 20 patients, 16 underwent autografting with PBPC alone. Fifteen patients engrafted neutrophils and platelets within 16 days. One patient had delayed engraftment associated with inadequate etoposide clearance. Statistical analysis showed a strong correlation between numbers of CFU-GM and CD34 positivity. The numbers of plastic-adherent P delta progenitor cells did not correlate with CFU-GM or CD34-positive cells. We conclude that the plastic-adherent P delta progenitor cell assay is capable of measuring primitive hematopoietic cells and that it may be useful for the investigation of primitive progenitors in PBPC harvests.