The most common and significant cause of disturbances in the normal intestinal microflora is the administration of antimicrobial agents. The microflora can be influenced by antimicrobial agents because of incomplete absorption of any orally administered antimicrobial agent, secretion of an antimicrobial agent in the bile, or secretion from the intestinal mucosa. Administration of these agents may seriously disturb the balance of the normal intestinal microflora. This disturbance can cause bacterial overgrowth and emergence of resistant microorganisms which may lead to serious infections and also encourage transfer of resistance factors among bacteria. The ecological effects of penicillins, cephalosporins, monobactams, carbapenems, macrolides, clindamycin, tetracyclines, nitroimidazoles and quinolones on the human intestinal microflora are presented in this review article.