The uptake of sulfonamides and sulfonamide potentiators by plasma albumin, red blood cells and hemoglobin of man, ox, rabbit and mouse has been determined. From the figures obtained the distribution of these compounds between cells, macromolecules and aqueous phases of blood has been calculated. In most cases more than 50% of total sulfonamide in blood is bound to plasma albumin. The time necessary for establishment of concentration equilibrium of the drugs investigated between erythrocytes and surrounding medium varies between a few seconds and several minutes, differences between the species being encountered. Hemoglobin binding of the drugs is much smaller than albumin binding. Nevertheless, drug concentration within the erythrocytes is generally higher than in the surrounding medium.