A laboratory technique for measuring the mobility of solutions on filter paper has been presented. Various exudates exhibited different mobilities on filter paper; this also applied to exudates of the same origin (pleura punctates). When diluted, the exudates exhibited increased mobility, indicating that the concentrations of various exudate components were of importance. The addition of cells (5 × 106 and 25 × 106 P 388 D cells/ml exudate) did not significantly change the mobility, whereas the mobility of protein fractions (human albumin and gamma globulin) was inversely related to the concentration of the solutions. These findings may he of significance to the reliability of present methods used for the quantitation of gingival exudation.