The origination of urinary calculi and the problems connected therewith are briefly discussed. Immuno-electrophoresis was found to be a suitable method for studying the urinary colloids. It was found that a certain fraction of the urinary colloids consists of proteins that are identical with serum proteins as regards migration velocities and serologically active groups. The following proteins are present in normal urinary colloids: albumin, an x-fraction (“séromucoïde acide”), one α 1-globulin, two α 2-globulins, one β 1-globulin and one γ-globulin, α 1- and α 2-lipoglobulins and α 2- and β 2 macroglobulins do not occur in urine. There was scarcely any difference between the urinary colloids of normal persons and those of patients suffering from oxalate calculi, as regards those components that also occur in serum. The results obtained in saturation experiments and direct electrophoresis were practically identical. If protein components that are not normally present (such as lipoglobulins and macroglobulins) are found in urinary colloids, it can be concluded that serum or blood has passed directly into the urinary ducts. Practically all the serum protein components that occur in urinary colloids are also found in the stone matrix of oxalate calculi. Serum proteins not normally present in urine could not be detected in the matrix. The authors consider it very likely that the presence of these serum proteins in the urine is due to their ability to pass through the glomerulus membrane. Macroglobulins, on the other hand, are unable to pass through this membrane.