Human fetal lung rudiments (8-12 weeks of development) undergo considerable growth upon microsurgical ectopic implantation in the xenograft-tolerant SCID mouse, and differentiate into a lung-like tissue that includes: (i) bronchial structures lined with pseudostratified, secretory, ciliated epithelium surrounded by smooth muscle and cartilage rings, (ii) submucosal glands, and (iii) alveolar sacs. Normal expression of the cystic fibrosis transmembrane conductance regulator (CFTR) protein was detected by immunostaining in those grafts, and similar differentiation was observed from either normal or cystic fibrosis (CF) fetal lung rudiments. Upon microinjection into human CF or normal lung grafts in SCID mice, beta-galactosidase-adenovirus gene constructs were efficiently transduced into epithelial and glandular cells. Such an in vivo replica of the human respiratory tissue may be a useful experimental model to study normal and pathologic lung development, and to assay candidate therapeutic gene constructs preclinically.