All tetracycline derivatives are bacteriostatics and their concentration in serum should not fall during the therapy below the generally accepted minimum therapeutic concentration of 0.5 to 1.5 mug/ml. Tetracyclines have a high affinity to form chelates with polyvalent metallic cations such as Fe+++, Fe++, Al+++, Mg++ and Ca++. Many of these tetracycline-metal complexes are either insoluble or otherwise poorly absorbable from the gastro-intestinal tract. Milk and other dairy products, antacids containing polyvalent cations, as well as various iron salts ingested simultaneously with tetracycline derivatives, might interfere with their absorption by 50 to 90% or even more. The severity of interaction depends both on the nature of the tetracycline derivative and of the cation, on the doses used, on pharmaceutical factors, and on time schedules in dosing. An interval of 3 hours between the ingestion of tetracyclines and cations prevents the interaction. The pharmacokinetic interactions in absorption of tetracyclines likely to be clinically significant in cases where the infecting pathogens are moderately resistant to tetracyclines and relatively high serum concentrations are needed for proper bacteriostasis.