In this work we intended to determine whether p53 and/or retinoblastoma (Rb) tumor suppressor genes are involved at specific stages in the process of in vitro human peripheral stem cell hematopoiesis. Mononuclear peripheral blood cells were depleted of adherent cells and T lymphocytes (A-T-PMCs). Cells were then cultured in semisolid medium, under conditions that favor the growth of specific progenitor cell types. A-T-PMCs were exposed to p53 and/or Rb sense, scrambled DNA and antisense oligodeoxynucleotides. p53 and/or Rb antisenses (but not their senses or scrambled DNA) treatment of A-T-PMCs resulted in a significant increase in the number of granu-locyte/macrophage colony-forming units (CFU-GM) in the presence of interleukin-3 (IL-3) and/or granulocyte/macrophage colony-stimulating factor (GM-CSF). After antisense treatment, blast forming units/erythroblasts (BFU-E) derived from A-T-PMCs cultured in the presence of IL-3 + erythropoietin (Epo) were also increased whereas colony forming units/erythroblasts (CFU-E) were not markedly affected in the presence of Epo only. Megakary-ocytic colony (CFU-Meg) formation from A-T-PMCs in the presence of interleukin-6 (IL-6) + IL-3 + Epo was also increased after antisense oligodeoxynucleotide treatment. These results are consistent with the hypothesis that p53 and Rb tumor suppressor gene products are involved in the control of distinct signal pathways in different peripheral progenitor cells.