THE USE of methyl cellulose solutions in ophthalmology is growing daily.1Because there is a variation in the concentrations recommended for use, as reported in the literature, my associates and I have tried to determine the concentration of methyl cellulose which would be most satisfactory and the proper composition of solutions of this substance. It also seemed advisable to investigate further the physical stability of solutions of methyl cellulose. Our findings are presented below. CONCENTRATION OF SOLUTION After trying the recommended 4,000-centipoise methyl cellulose in various strengths, I found that the 0.33 + % solution was the most satisfactory as a tear substitute. Concentrations of 0.25, 0.5, and 1% proved either too thin or too thick. THE VEHICLE—pH AND TONICITY The methyl cellulose was tried in various vehicles, as it is necessary to make the solution isotonic because the large methyl cellulose molecule exerts practically no osmotic