PAST work on monolayers of gelatin on water has generally led to the conclusion that it is not suitable for such studies because of its high solubility1. This difficulty has been overcome with other proteins by spreading on strong solutions of ammonium sulphate2, and we have found that stable monolayers of collagen can be formed on 30 per cent ammonium sulphate solution (pH 4) by spreading from formic acid solution. The surface pressure (π) and surface potential (ΔV) have been studied over a pressure-range of 0·01-30 dynes/cm. When the area (A) is greater than about 5 m.2/mgm. the film is heterogeneous, large fluctuations of ΔV being observed as the ionizing air-electrode is moved over the surface, although no constant surface vapour pressure was found. As the film is compressed to areas smaller than about 5 m.2/mgm. it becomes homogeneous and ΔV.A is constant. On further compression, two critical areas are recognized : (1) A1 (1·8 m.2/mgm.), below which values of ΔV.A fall, indicating a change in orientation, accompanied by a decrease in compressibility; (2) A2 (1·3 m.2/mgm.), which is derived by extrapolating the linear portion of the π—A curve (between 1·0 and 0·2 m.2/mgm.) to zero pressure.