A substantial body of evidence accumulated in recent years indicates a protracted delay in immune reconstitution following autologous stem cell transplantation. In order to investigate the cellular basis of this phenomenon, peripheral blood mononuclear cells were studied from recipients of autologous stem cell transplantation for solid tumors and hematological malignancies. On stimulation with phytohemagglutinin and phorbol 12-myristate 13-acetate, transplant-derived peripheral blood mononuclear cells demonstrate statistically significant depressed production of interleukin 3 (IL-3), IL-4, granulocyte-macrophage-colony-stimulating factor, and gamma-interferon as compared to normal controls, during the first 6 months following engraftment, which recover to normal levels 6 months or more posttransplant. When the overall group of transplant recipients is compared to the control group, there is a statistically significant lower production of IL-2. In addition, no differences were observed regardless of the source of the engrafted stem cells, whether from bone marrow alone (autologous bone marrow transplantation), from peripheral blood stem cells alone, or from a combination of autologous bone marrow transplantation and peripheral blood stem cells. The defect persisted past 6 months postengraftment. Transplant-derived peripheral blood mononuclear cells were stimulated with combinations of either phytohemagglutinin plus the calcium ionophore A23187, thereby circumventing the requirement for accessory cell function, or with phorbol 12-myristate 13-acetate plus anti-CD28 monoclonal antibody, mimicking the CD28-B7 cell surface-ligand interaction capable of triggering and stabilizing IL-2 gene transcription. In both situations, decreased production of IL-2 as compared to controls was observed in individuals within 6 months of transplantation. Quantitative polymerase chain reaction indicates that decreased transcription of IL-2 mRNA following transplantation is not due solely to a decrease in the absolute numbers of CD4+ T-cells but is secondary to reduced numbers of transcript copies per cell. Production of IL-10 was found to be decreased regardless of whether the autologous graft was of bone marrow or peripheral blood origin. These findings are consistent with the conclusion that: (a) multiple dysregulations exist in the production of cytokines important in immune homeostasis; (b) a defect occurs at or prior to the level of transcription of IL-2 mRNA; (c) IL-10 does not play a direct role in the pathogenesis of posttransplantation immunosuppression; and (d) there is no evidence that peripheral blood stem cells may be superior to bone marrow-derived stem cells in accelerating immune reconstitution.