no more than just "pasteurized" - that is, do not use "ultra-pasteurized" milk or it will not work!
I followed the ridiculously thorough examples at Instructables.com's page on how to make mozzarella. The one and only suggestion I might make for an otherwise excellent tutorial has to do with their Step 16, in which they tell you to heat and knead the cheese until it is pliable. I kept on kneading and heating, and all I got was crumbly cheese. Undaunted, I looked up troubleshooting suggestions on what to do when your mozzarella is in this state, and found the answer: to become stretchy, the mozzarella must be at least 138°F. Instructables.com never mentions this, but keep it in mind when you use their otherwise excellent website to make your own.