Platelet engraftment, the time course and magnitude of platelet recovery (PR) post-transplant, is imprecisely defined but is most often reported as the time to transfusion (tx) independence and/or a platelet count > or = 20,000/microl. While correlations between engraftment time for granulocytes (PMN) and the dose of CD34-positive cells per kilogram are established, such associations have not been established for platelet engraftment. The objective of this study was to quantify subpopulations of CD34-positive cells in peripheral blood stem cell (PBSC) collections of normal, colony-stimulating factor-granulocyte) (G-CSF) primed donors that might represent megakaryocyte (MK) precursors, and to determine whether there is a statistical association between the dose transfused and the time course of the recovery. Based on previously published data of the sequential expression of CD34, HLA-DR, and CD61, among others, during MK maturation, a combination of corresponding antibodies for the detection of various antigen coexpressions by flow cytometry fluorescence-activated cell sorting [FACS] was chosen. CD34-positive cells were further subdivided into CD34++ (bright) and + (dim). Ploidy of density-gradient separated cells was examined in subsequent donor samples by FACS. For the entire group of patients, there was no strong correlation between any of the studied subpopulations and time to PR. Only in a selected groups of patients whose platelet counts showed a sustained increase during the first 6 days after engraftment, there was a weak correlation between the time to PR and the quantity of CD34+/+CD61+ (r = -0.57) and CD34++HLA-DR-CD61+ (r = -0.62) cells infused. The magnitude of platelet production in these pt., a product of the peripheral blood platelet count and the patient's blood volume, was correlated with the time to PR (r = -0.73). We conclude from this study that subpopulations within CD34+ cells are making some contribution to PR in allogeneic peripheral blood stem cell transplantation, but the correlations are not sufficiently strong because there are probably too many unpredictable and unknown variables in the allogeneic setting that influence the pattern of engraftment.