The principal active components of Penicillium are listed in the following table:
Other minor active components of Penicillium include penicillin O, penicillin U1 and penicillin U6. Other named constituents of natural Penicillium, such as penicillin A, were subsequently found not to have antibiotic activity and are not chemically related to antibiotic penicillins.The precise constitution of the penicillin extracted depends on the species of Penicillium mould used and on the nutrient media used to culture the mould. Flemings original strain of Penicillium rubens produces principally penicillin F, named after Fleming. But penicillin F is unstable, difficult to isolate, and produced by the mould in small quantities.The principal commercial strain of Penicillium chrysogenum (the Peoria strain) produces penicillin G as the principal component when corn steep liquor is used as the culture medium. When phenoxyethanol or phenoxyacetic acid are added to the culture medium, the mould produces penicillin V as the main penicillin instead.6-Aminopenicillanic acid (6-APA) is a compound derived from penicillin G. 6-APA contains the beta-lactam core of penicillin G, but with the side chains stripped off; 6-APA is a useful precursor for manufacturing other penicillins. There are many semi-synthetic penicillins derived from 6-APA and these are in three groups: antistaphylococcal penicillins, broad-spectrum penicillins, and antipseudomonal penicillins. The semi-synthetic penicillins are all referred to as penicillins because they are all derived ultimately from penicillin G.

Penicillin units
One unit of penicillin G sodium is defined as 0.600 micrograms. Therefore, 2 million units (2 megaunits) of penicillin G is 1.2 g.
One unit of penicillin V potassium is defined as 0.625 micrograms.
Therefore 400,000 units of penicillin V is 250 mg.The use of units to prescribe penicillin is a historical accident and is largely obsolete outside of the US. Since the original penicillin was an ill-defined mixture of active compounds (an amorphous yellow powder), the potency of each batch of penicillin varied from batch to batch. It was therefore impossible to prescribe 1 g of penicillin because the activity of 1 g of penicillin from one batch would be different from the activity from another batch. After manufacture, each batch of penicillin had to be standardised against a known unit of penicillin: each glass vial was then filled with the number of units required. In the 1940s, a vial of 5,000 Oxford units was standard, but the depending on the batch, could contain anything from 15 mg to 20 mg of penicillin. Later, a vial of 1,000,000 international units became standard, and this could contain 2.5 g to 3 g of natural penicillin (a mixture of penicillin I, II, III, and IV and natural impurities). With the advent of pure penicillin G preparations (a white crystalline powder), there is little reason to prescribe penicillin in units. The "unit" of penicillin has had three previous definitions, and each definition was chosen as being roughly equivalent to the previous one. Oxford or Florey unit (1941). This was originally defined as the minimum amount of penicillin dissolved in 50 ml of meat extract that would inhibit the growth of a standard strain of Staphylococcus aureus (the Oxford Staphylococcus).