Penicillin, introduced in the 1940s, was the first antibiotic for clinical use. Since that time, a variety of penicillins have been developed. The penicillins are also known as the beta-lactam antibiotics because they contain a beta-lactam ring that is necessary for their anti-microbial activity. They are bactericidal and destroy numerous gram-positive and some gram-negative bacteria. The penicillins have been placed in four groups depending on their chemical structure and the type of bacteria they are able to kill: natural penicillins, penicillinase-resistant penicillins, aminopenicillins, and extended-spectrum penicillins. It should be noted that there is a group of combination drugs that contain penicillins and beta-lactamase inhibitors. The beta-lactamase inhibitors prevent destruction of penicillin from enzymes and enlarge the spectrum of antimicrobial activity of penicillin. These combination drugs are listed in the drug table on page 7. Penicillins in general have few adverse effects and are well tolerated. They are well distributed to most body tissues and fluids and are eliminated by the kidneys.