IT has long been known that the surface electrical potential of a protein monolayer at the air/water interface is dependent on the pH and the nature of the ions present in the aqueous sub-solution. There is no record, however, of a comprehensive investigation of the variation of protein surface potential with the pH of the sub-solution. This information, were it available, would be a ‘titration curve’ of a protein monolayer, and would shed light on the nature and extent of the ionizing groups of the protein when spread as an insoluble monolayer. We wish to report preliminary results of such a surface titration curve for the protein bovine serum albumin.