Better to avoid using this term. Open symbols are controls. Closed symbols are in the presence of probenecid.
Squares represent CSF and circles represent serum penicillin G concentration.
Think about maximum achievable plasma concentrations and reasons for needing to vary absorption rates. Name two acid resistant penicillins, one each from the natural penicillins and one from the aminopenicillins. You should know that methicillin is the classical anti-staphylococcal pencillin derivative. Yet staphylococci can become resistant to it. This is an example of the kind of mechanism pharmacologists love to talk about.