1. A static method of measuring the viscoelasticity of spread monolayers has been presented, and a rheological study has been carried out with the monolayers of proteins and synthetic polypeptides. 2. With ovalbumin films spread on 0.1 N hydrochloric acid and 5% ammonium sulfate solution, it has been found that an instantaneous elasticity (Hookean), followed by retarded elasticity and flow, appears at unexpectedly large areas, such as 5 sq. m./mg. Surface shear modulus of the instantaneous elasticity increased linearly with the decrease of surface area, exhibiting several breaks in the slope of the curve. 3. The films of serum albumin and hemoglobin did not exhibit any viscoelasticity until they were compressed down to the area below 1.0 sq. m./mg., where the films became viscous. Further compression down to below 0.8 sq. m./mg, rendered these films viscoelastic. At areas corresponding to such changes of film state, the compressibility-area curve exhibited breaks. 4. Poly- DL-α-aminocaproic acid and poly-γ-methyl L-glutamate showed an instantaneous elasticity (non-Hookean) at the areas larger than that of the close-packed monolayer. The monolayers of poly-ε-aminocaproic acid behaved like an ideal elastic body. 5. Concerning the form of the force-area eurves, an interesting fact has been found: The films which do not give any instantaneous clastieity have no break point in their force–area curves; whereas, the films which exhibit the distinct instantaneous elasticity show the breaks or plateaus in their force–area eurves.