The regulations in different countries on the toxicological testing of antimicrobial agents are similar and harmonized on an international basis. Existing requirements cover both the standard investigations performed with any other new class of therapeutic drug and investigations necessary due to the specific features of anti-infective agents. Such features are the therapeutic target (the microorganisms), the need to provide adequate treatment of patients even during clinical trials, and the potential of the drug to induce certain adverse reactions. The combination of toxicity tests used will be determined by the drug, its class and current knowledge.