The membrane permeability of a series of reversible cross-linking reagents which are diazide tartarate derivatives has been compared with that of dimethyl-3,3'-dithiobispropionimidate (DTBP). The diazide tartarate derivatives tested include tartryl-diazide (TDA), tartryl-di(glycylazide) (TDGA), tartryl-di(beta-alanylazide) (TDAA), tartryl-di-(gamma-aminobutyrylazide) (TDBA), tartryl-di(epsilon-aminocaproylazide) (TDCA). TDA, which has the shortest chain length of the diazide tartarate derivatives tested, proved to be readily permeable through the erythrocyte membrane. When added at equal concentration to unsealed ghosts, TDGA was at least as reactive as DTBP in its ability to cross link the internally displayed proteins 1, 2, 4.1, 4.2, and 6. Treatment of resealed ghosts by DTBP produced oligomeric complexes of these proteins plus apparent homooligomeric complexes of hemoglobin. TDGA at the same concentrations did not cross-link any of these components, indicating its membrane-impermeable nature. As the chain length of the homologous series increased from TDGA to TDCA, the cross-linkers became increasingly permeable through the erythrocyte membrane.