Purpose:The purpose is to determine the immunological effects of recombinant human interleukin (rhIL)-12 therapy after autologous stem cell transplantation.Experimental Design: Twelve patients (8 non-Hodgkin's lymphoma, 2 Hodgkin's disease, and 2 plasma cell myeloma) were treated with rhIL-12 by bolus i.v. injection in doses of 30, 100, or 250 ng/kg starting at a median of 66 days posttransplant. Immunological assays were performed using serum and peripheral blood mononuclear cell (PBMC) samples obtained on study.Results: Dose-dependent increases in the total lymphocyte count occurred during rhIL-12 therapy. The absolute number of peripheral blood CD4 T cells increased up to 16.3-fold, CD8 T cells up to 20.5-fold, B cells up to 11-fold, and natural killer (NK) cells up to 12.3-fold during rhIL-12 administration and returned to pretreatment baseline levels after discontinuation of rhIL-12. CD56 bright NK cells expanded dramatically in the blood of a patient with baseline lymphopenia before rhIL-12 therapy. In vitro proliferation of patient PBMCs in response to IL-12 was indistinguishable from that of PBMCs obtained from healthy control subjects. Moreover, spontaneous in vitro proliferation of patient PBMCs increased significantly during rhIL-12 therapy. Increased levels of IFN-␥ and IL-18 were detected in the serum of patients treated in the 100 and 250 ng/kg dose cohorts during the first multiple dose cycle.