What name was later used for Fleming's 1928 discovery also called penicillin G?

Fleming's 1928 discovery was later called benzylpenicillin, also known as penicillin G.

Fleming observed that a Penicillium mold released an antibacterial substance into a culture plate and named the substance penicillin. The early material was a crude, unstable mixture rather than a pure, standardized drug. Later researchers isolated and characterized different penicillin compounds, and the form known as benzylpenicillin became the classic penicillin G used in medicine.

The name helps distinguish this drug from other antibiotics. Tetracycline, chloramphenicol, and streptomycin are separate antibacterial agents discovered or developed in different historical contexts. “Penicillin” can refer broadly to the family of beta-lactam antibiotics, while benzylpenicillin identifies a particular natural penicillin. The letter G is another accepted name for the same compound, not a reference to Fleming's initial laboratory designation.