The Beta-Lactam ring is the four membered ring enclosed in the blue circle. The red circle shows the carboxylic acid group which is another common structural feature in Penicillin and Penicillin derived antibiotics. Penicillins such as Penicillin G, Penicillin V, and Ampicillin have essentially the same basic structure and differ only in the R-groups bound to the carbonyl group shown at the top left of this diagram. The gray balls indicate carbon atoms, the red balls indicate oxygen atoms, the blue balls indicate nitrogen atoms, and the greenish blue ball is a sulfur atom. The hydrogens in this molecule are not shown.