User blog comment:Hardly a Notion/Jax/@comment-3554826-20111006044914

Sword of the Divine is a must for any auto-attacker to counter a fed Jax. You must not auto-attack him in team fights without, or else you'll be giving him lots of AoE stuns while dealing no real damage because of his dodges. Exhaust him when he leaps into someone, even if you're focusing someone else.

Items and skills that deal % of health as damage ar nice too. Jax usually has a large health pool because of his passive, but won't be getting magic resistance before he gets his core items.

But the main counter against Jax is to do not allow him to get fed/farmed. He's rather weak at early game, so you should gank and zone his brains out before he gets level and items. An underfarmed Jax shouldn't be too scary.