Amoxicillin (α-amino-p-hydroxybenzylpenicillin) is a new semisynthetic penicillin structurally related to ampicillin. The antibacterial spectrum of amoxicillin includes streptococci, pneumococci, penicillin-sensitive staphylococci, Haemophilus infiuenzae, Neisseria meningitidis, Neisseria gonorrhoeae, Escherichia coli, Proteus mirabilis, and Salmonella species. The level of activity of amoxicillin is generally similar to that of ampicillin. Amoxicillin is more active than ampicillin against experimental infections of mice with gram-positive and gram-negative bacteria. The superior in vivo activity of amoxicillin cannot be explained solely on the basis of increased oral absorption. Bactericidal action measured in a mouse thigh infection model is more rapid with amoxicillin. Blood levels of amoxicillin in man are twice those achieved with equivalent doses of ampicillin. Peak serum concentrations are proportional to the dose, and 58%–68% of a dose is recovered unchanged in the urine. Absorption of amoxicillin is not greatly influenced by the presence of food. As with other orally-administered penicillins, higher and more prolonged levels of amoxicillin in blood are obtained when probenecid is used.