SummaryTetracycline, doxycycline and minocycline are true broad spectrum antibiotics which remain the primary agents againstChlamydiae, Rickettsiae, Brucella, Pasteurella, Borrelia, Mycoplasma, Ureaplasmaand some additional specific agents of infectious diseases. The usefulness of these antibiotics against common gram-positive and gram-negative bacteria is dependent on the incidence of acquired resistance, which is mostly plasmid-mediated. Among gram-positive organisms tetracycline resistance is widespread in streptococci and coagulase-negative staphylococci. However, minocycline is generally active on the latter organisms. The activity against gram-negatives is unpredictable at present and susceptibility tests arc therefore mandatory. Some species, especiallyE. coliare more susceptible to minocycline than to the other compounds. Tetracyclines are frequently active on non-fer-mcntative bacilli other thanPseudomonas aeruginosa.They are primary agents for en-teropathogenic organisms, but here also, susceptibility tests are mandatory in order to exclude acquired resistance. The same holds true for many fastidious organisms such asHaemophilusand gonococci. From the other side, no acquired resistance has been demonstrated up to the present inBranhamella catarrhalisandChlamydiae.Although tetracyclines arc active on many anaerobic organisms, they arc no longer indicated against such bacteria.