It is not an SI unit because it contains the non-SI unit "litre". The SI unit of concentration is kilogram per cubic metre, which is numerically equivalent (1 g/L = 1 kg/m³).
SI prefixes are often applied so that there are units like milligrams per litre (mg/L). When measuring concentration in water, parts per million is an older expression of mg/L, since one liter of water under standard conditions weighs one kilogram or one million milligrams.
Milligram per litre is often used in medicine and is also used among prescriptions. For example you may be given a solution which involves one substance and another substance and one of the substances involves adding water then it would state: "10 mg/L water and ..." Similarly milligrams per deciliter (mg/dl or mg%) is also a common medicinal unit.
Reports from University of Queensland, Australian Institute for Bioengineering and Nanotechnology describe recent advances in vaccines.(Report)
Dec 15, 2010; Fresh data on vaccines are presented in the report 'Microbial production of virus-like particle vaccine protein at gram-per-litre...
Publication No. WO/2009/082939 Published on July 9, Assigned to ROTAM AGROCHEM INTERNATIONAL for Aqueous Suspension Concentrate Composition (Chinese Inventor)
Jul 14, 2009; GENEVA, July 14 -- James T. Bristow, China, has developed an aqueous suspension concentrate composition of a crop protection...