The uptake of methylene blue (MB), and toluidine blue O (TBO) by bovine pulmonary arterial endothelial cells grown on microcarrier beads was detected as a decrease in the concentration of dye in the medium after these thiazine dyes were added to the medium surrounding the cells. Because the reduced forms of these dyes are much more lipophilic than the oxidized forms, we considered the possibility that reduction of the dyes at the cell surface might have preceded the uptake by the cells. Therefore, we studied the ability of the cells to reduce a toluidine blue O-polyacrylamide polymer (TBOP), which was too large to enter the cells in either the oxidized or reduced form. The TBO moieties of the polymer were reduced by the cells, indicating that the dyes did not have to enter the cells to be reduced and that reduction can occur at, or near, the cell surface. The rate of TBOP reduction was about the same as the rate of uptake of the monomeric dyes, indicating that the cell surface reduction mechanism had a sufficient capacity to account for the monomer uptake by the cells. We also found that ferricyanide ion, which also did not permeate the cells, was reduced by the cells and that external ferricyanide inhibited the monomeric MB uptake. Thus the results with ferricyanide were also consistent with the concept that the monomeric thiazine dyes are reduced at the cell surface before the more lipophilic reduced forms are taken up by the endothelial cells.