What it is and where it comes from
Lysozyme is a natural enzyme found in egg white. It is used as a preservative, mainly in cheese, where it stops gas-forming bacteria.
Where you find it
Hard cheeses such as Grana Padano.
What to look for on the label
Lysozyme itself is halal. The cheese it is in still has the rennet question, so check the cheese too.
This is a guide, not a fatwa. Recipes change, so check the label, look for halal certification, and ask your local imam where scholars differ.