The surface activity and enzymic properties of the factor F1, the catalytic moiety of Streptococcus faecalis H+-ATPase, has been studied at the air-water and phospholipid-water interfaces. F1 does not interact with the monolayer phospholipids, hence its adsorption on a biological membrane must be due mainly to its recognition of proteins of the hydrophobic complex. The dimensions of the F1 molecule at the air-water interface have been estimated. In the presence of Mg2+, base area is S = 1.8 . 10(4) A2, height h = 27 A. Bearing in mind the size of a globular subunit, it follows from the measurements that the major F1 subunits should all lie in the same plane. The ATPase activity of F1 at the interface is inversely proportional to the monolayer density. With low density monolayer, the specific ATPase activity is higher at the interface than in the bulk of the solution. Adsorption of F1 at the interface shifts the isoelectric point of tiscussed relative to the proton-active transport mechanism.