A strong immune response against transgenic cells is one important limitation for long-term expression after adenoviral gene transfer in mammals. Continuous pharmacological immunosuppression has been shown to ameliorate immune reactions and to prolong reporter gene expression. In this study, we explored the effect of short-term immunosuppression for long-term gene expression and its impact on antibody formation. Immunosuppression with FK 506 (1 mg/kg/day), cyclosporin A (20 mg/kg/day) and 15-deoxyspergualin (10 mg/kg/day) was performed in NMRI mice. Expression of the reporter gene human alpha-1-antitrypsin (hAAT) and antibody formation was monitored for 7 months. A 5-day course of 15-deoxyspergualin (15-DSG) markedly slowed the decline of reporter gene expression and a positive effect was still detectable 200 days after gene transfer. At the same time, antibody production was reduced by 50-60%. Continuous treatment with 15-DSG (10 mg/kg twice weekly) led to a further small increase of gene expression but reduced antibody formation by 80-90%. A short course of FK 506 and cyclosporin A (CsA), had conferred a negative effect on gene expression. Both groups showed an even faster reduction in gene expression compared with the control group. The results of this investigation suggest that 15-DSG could serve as an effective supplement for viral gene therapy protocols.