AbstractA study of the film molecular weight of various proteins is made.Two ranges of surface pressure are examined using in the 0,001‐0,02 dynes/cm range a modified Guastalla balance, and, in the 0,02‐0,6 dynes/cm range a Wilhelmy plate method or a Marcelin balance.Usually the molecular weights found in both ranges of pressure are in agreement although in some cases those found in the 0,02‐0,6 dynes/cm range are not correct due to the shape of the isotherm.The influences of several variables: pH, ionic strength, surface active agent, temperature are investigated. Special mention is to be made of the influence of the substrate temperature on the isotherm of eggalbumin.On the basis of our results, proteins can be classified in two groups: firstly, we find all the proteins giving, when spread at pH 2, a film molecular weight of the same order of magnitude of the osmotic or ultracentrifugation‐diffusion molecular weight; secondly, we have the proteins which give a film molecular weight lower than that found in solution.A discussion of the influence of the electrostatic factor and of the flexibility of the protein pattern is made.Finally attention should be paid to the necessity of considering an enthalpy factor in any theory of protein monolayer.