To explore the feasibility of ex vivo lymphocyte gene therapy for mild Hunter syndrome (mucopolysaccharidosis type II), we evaluated retrovirus-mediated gene transfer of the iduronate-2-sulfatase (IDS) coding sequence into peripheral blood lymphocytes from enzyme-deficient individuals (PBL(MPS)). Moloney murine leukemia virus-derived retroviral vectors were constructed by inserting the IDS cDNA under transcriptional regulation of the long terminal repeat (LTR) (in vector L2SN) or the cytomegalovirus (CMV) early promoter (vector LNC2). High-titer virus-producer cells were generated using amphotropic PA317 packaging cells. After 3 days of in vitro stimulation of T lymphocytes with anti-CD3 antibody and interleukin-2 (IL-2), PBL(MPS) were transduced once on each of the next 3 days. Seven to 21 days later, cultured PBL(MPS) were evaluated for gene transfer and IDS specific activity. Heterogeneous populations of L2SN-transduced PBL(MPS) had high levels of IDS enzyme activity (456 U/mg per hr ± SD 292) despite a gene transfer efficiency of 5% or less. Owing to overexpression of IDS in that percentage of PBL(MPS) successfully transduced, IDS activity was increased above the deficiency found in patients with Hunter syndrome (<20 U/mg per hr) to a level comparable with that of normal individuals (mean activity of uncultured normal leukocytes 807 U/mg per hr; SD 252). Reduced 35SO4-glucosaminoglycan (GAG) accumulation was observed in PBL(MPS) that had been transduced with L2SN, or when PBL(MPS) were grown in medium that had been 'conditioned' by growth of L2SN-transduced cells. This latter result indicated that metabolic cross-correction occurred by means of intercellular enzyme transfer. These studies of retrovirus-mediated expression and metabolic correction, finding near-normal levels of IDS in cultured PBL(MPS) and metabolic correction, demonstrate the potential for treatment of mild, nonneuropathic Hunter syndrome by means of ex vivo lymphocyte gene therapy.