1. The stability of monolayers of a highly unsaturated yeast lecithin labelled with (32)P has been investigated by a surface radioactivity technique. 2. Lecithin films on distilled water at all surface pressures between 6 and 48dynes/cm. were completely stable on rapid perfusion of the subphase and on addition of ionic amphipathic substances to the film. 3. Ultrasonically treated lecithin added to the subphase caused a slow loss of surface radioactivity but little pressure change. 4. The addition of proteins to the subphase caused negligible changes in the film even when conditions were favourable for electrostatic heterocoagulation and penetration. 5. Lecithin films were not hydrolysed by a strongly acid subphase at room temperature. The very low rate of hydrolysis produced by alkali was proportional to the subphase OH(-)ion concentration: the apparent activation energy and temperature coefficient (Q(10)) of the reaction were 14250 cal. and 2.37 respectively. 6. Alkaline hydrolysis of lecithin monolayers was markedly stimulated by adding methanol (10-20%, v/v) to the subphase. The addition of ionic amphipaths to the monolayer had the expected type of effect on the hydrolysis rate, but its magnitude was far less than that suggested by an application of the Poisson-Boltzmann equation for ion distribution at a charged interface (Davies & Rideal, 1963).