Polyjuice potion

12 lacewing flies that have been stewed for 21 days
1 ounce of crude antimony
4 leeches that have been "unsucculated" (possibly meaning their suckers have been removed)
6 scruples of fluxweed that were picked at full moon
3 drachms of pulverised Sal Ammoniac
Pulverised blades of knotgrass
1 pinch of powdered horn of a Bicorn that has been "lunar extracted" (possibly meaning that, like the fluxweed, its collection time coincided with certain lunar conditions)
Filings and rasplings of Saltpeter, Mercury and Mars ("filings of Mars" may mean iron filings, as the symbol for the planet Mars, ♂, is also used to signify the element of iron )
Shredded dried skin of a Boomslang
Extract of The-Transfigured-Being-To-Be (which means a piece of the person one wants to change into, typically hair)

Most of the ingredients are available in the students' cupboard at Hogwarts, although some of them are only available in Severus Snape's private stores. It takes at least one month to brew properly.