Uptake of tetracycline (tc), 2-tetracyclinonitrile (CN-tc), and 9-(N,N- dimethylglycylamido)-6-demethyl-6-deoxytetracycline (DMG-DMDOT) by liposomes containing Tet repressor (TetR) and by Escherichia coli cells overexpressing TetR was examined. TetR specifically binds to tetracyclines, enhances their fluorescence and thereby allows selective detection of tetracyclines that have crossed the membranes. Analysis of the diffusion of tc and DMG-DMDOT into liposomes yielded permeation coefficients of (2.4 ± 0.6) x 10-9 cm·s-1 and (3.3 ± 0.8) x 10-9 cm·s-1, respectively. Similar coefficients were obtained for uptake of these tetracyclines by E. coli, indicating that diffusion through the cytoplasmic membrane is the rate- limiting step. The permeation coefficients translate into half-equilibration times of approximately 35 ± 15 min and explain how efflux pumps can mediate resistance against tetracyclines. Furthermore, diffusion of CN-tc into liposomes was at least 400-fold slower than that of tc, indicating that the carboxamide group at position C2 is required for efficient permeation of tc through lipid membranes and thereby explaining the lack of antibiotic activity of CN-tc.