Antibiotics are known as miracles of modern medicine, because their discovery changed people’s lives. Their ability to kill pathogens, and not to kill the host makes them useful in fighting many diseases. It should be emphasized that antibiotics act on bacteria, but not on viruses. One of many classifications of antibiotics is: macrolides, aminoglycosides, tetracyclines and chloramphenicol. All of them kill bacteria, by inhibiting the synthesis of bacterial proteins. Erythromycin is one of the macrolide antibiotics synthesised by the bacteria Streptomyces erythreus. A macrocyclic ring and two molecules of sugar, desosamine and cladinose are part of its structure. Erythromycin, alike other macrolide antibiotics, inhibits protein synthesis by binding to the 23S rRNA molecule in the 50S-subunit of the bacterial ribosome. It affects protein synthesis by inhibiting the activity of peptidyl transferase, which is responsible for making peptide bonds between amino acids, hence preventing peptide bond formation. Resistance of microorganisms to antibiotics is constantly increasing. Resistant microorganisms disable the entry of erythromycin into the cell, cause changes on the ribosomes and hydrolyse erythromycin with esterases. The amynoglicoside streptomycin, was the first effective medication against tuberculosis. It is synthesised by the bacteria Streptomyces griseus, but because of a high level of resistance it is rarely used. In its structure streptomycin has the hexose aminocyclitol streptidin. Two amino sugars, sreptosis and Nmethyl-L-glucosamine, are via a glycosidic bridge bonded to the hexose. Streptomycin inhibits the initiation of the peptidyl chain and causes missreading of the codon. Anaerobic bacteria and resistant, mutant bacteria are resistant because they don’t have a transport system for streptomycin and other aminoglycosides. The resistance of the bacteria Mycobacterium tuberculosis is increasing. Tetracycline is synthesised from chlortetracycline, isolated from the bacteria Streptomyces aureofaciens. Tetracycline inhibits the synthesis of bacterial proteins by binding to the 30S-subunit of the ribosome. Tetracycline also inhibits the binding of aminoacyl-tRNA to the acceptor site in the ribosome, inhibiting the elongation of the polypeptide chain. Resistance to tetracyclines is expanding slowly and progressively. Over the years many microorganisms have become resistant because of the frequent use of these antibiotics. Resistance is one of the main problems considered by scientists nowadays. Resistance of microorganisms is increasing, whereas there are fewer new antibiotics. The cause of that is a frequent usage of antibiotics even for those inflammations that do not require such treatment.