Penicillin is white or slightly yellowish white crystalline powder.

All the natural penicillin is dextrorotatory.

Penicillin is only sparingly soluble in water.

Penicillin is hydrolysed by alkaline medium to yield penicilloic acid, which loses carbon dioxide to form penicilloic acid.

If the hydrolysis of penicillin is carried out in acidic medium, the amide side chain is involved with the opening of β-lactam ring. This causes loss of its activity. Due to this reason, natural penicillins are not very much eff ective when given orally. Further, the stomach juices contain the enzymes penicillinases. These enzymes also cause hydrolysis of penicillin, resulting in the opening of the lactam ring, followed by the loss of activity.