The effect of three biopolymers on the mass transport of fluorescein esters into cultured epithelial cell monolayers was determined. The method involved measurement of the rate of accumulation of fluorescein after extracellular introduction of its relatively nonfluorescent ester derivatives. Calibration of the intracellular fluorescence along with cell enumeration allowed estimation of the molar velocity of uptake on a per cell basis. Fick's law with modified Michaelis—Menten kinetics provided a kinetic description of the sequential transport and cellular metabolism and permitted estimation of the permeability, maximum velocity, Vm, and apparent Michaelis—Menten constant, Km. From the uptake studies, the calculated Vm and Km decreased with increasing chain length of the ester which is consistent with deacylation of the enzyme being the rate limiting step of the enzymatic conversion. The estimated permeabilities of the controls were variable, in part due to the insensitivity of the functional relationship between the estimated parameters. All biopolymers caused a reduction in the calculated permeability, with guar gum and xanthan gum being more effective at a higher concentration than bovine submaxillary mucin. The results support the conclusion that polymers reduce the rate of mass transport through an increase in the resistance of the diffusional boundary layer.