12 day old kitten had severe swelling of the upper left arm. The next day the area around the swelling lost all hair and white puss was visible. Most of the puss was removed and the wound was treated with antibiotic sav.

Any idea what this could be?

Yikes, I have no idea but your description makes it sound pretty serious... has the kitten been looked at by a vet?

Hi,
I agree with your previous answer; I don't know either what this could be.
This is a great time to take it to a Vet!
Find out, before it might be too late.
Best of luck.

Could be a bite from mamma cat or a scratch, could be almost anything, If it does not heal on it's own take the cat to a vet.