Although contractile interstitial cells (CIC) in the alveolar septum have been suggested to be involved in hypoxic pulmonary vasoconstriction (HPV), direct demonstration of cellular contraction under hypoxia has been lacking. To achieve this, we purified CIC from collagenase-dissociated bovine lung cells and examined the response of these cells to hypoxia. Prostaglandin (PG) F synthase served as a marker of CIC, and the isolated PGF synthase-positive cells were shown to preserve the ultrastructural features characteristic of CIC, most notably bundles of microfilaments. Isolated CIC seeded onto collagen gel disks became embedded and formed a lattice network with collagen fibrils. Exposure of these CIC-bearing gels to hypoxia (PO2 = 20-40 Torr) evoked a reversible reduction in gel volume, as assessed by measuring the surface area of the gel disks photographically. Thus CIC were shown to contract under hypoxia, providing the supportive evidence for the involvement of CIC in HPV.