The permeability of the membrane of phagocytic leukocytes (WBC) to the non-complexed nitro-blue tetrazolium (NBT) cation has not been well explored. In this study both complexed and non-complexed (or free) NBT have been incubated with viable cells. The entry of NBT into the WBC was determined by both colorimetric and infrared analyses. Very little free NBT enters the viable WBC compared with NBT complexed with serum factors. NBT entry was also studied in non-viable WBC by use of the infrared system. The entry of NBT into the non-viable cells was independent of serum complexing factors. Much larger quantities of both free and complexed NBT enter non-viable cells than enter viable WBC. It is concluded that the membrane of the viable WBC is quite impermeable to the free NBT cation. Ultrastructural studies suggest that the gaps in the plasma membrane of non-viable cells could allow the passage of relatively large amounts of both free and complexed NBT.