Methylcellulose, injected intravenously in dogs, altered the surface properties of blood plasma, as measured both in the fluid and clotted state. In fluid plasma, it altered the pattern of the surface tension-time curve, instituting the following changes: a reduction in the dynamic value of surface tension, a progressive rise in value contrasted to the normal decline, an elevation of the final static level above the pre-injection value. In clotted plasma, methylcellulose converted the normal water repellent surface to one that was water attracting, or hydrophilic. There was no change of coagulation time in glass, but possibly some prolongation in siliconed tubes. No surface tension or hydrophilic changes were induced by the other colloids tested; namely, dextran, polyvinylpyrrolidone and carboxymethylcellulose. The physical changes induced by methylcellulose are of a type that might favorably influence clot structure and adhesion.