Point72's Steve Cohen. Steve Marcus/Reuters In a filing out on Tuesday after the market closed, hedge fund manager Steve Cohen revealed an 8.3% ownership stake in tiny company Celator Pharmaceuticals.

You might recognize the name. Celator shares jumped 432% in trading on Tuesday after announcing a successful test of its new drug targeting a form of leukemia.

Through his family-office fund, Point72 Asset Management, Cohen acquired just over 2.8 million shares on Monday after the news of the successful trial was released.

Tuesday's disclosure came in a 13G filing, required whenever an investor takes a stake in a company equal to more than 5% of a company.

So while this isn't a huge part of Cohen's portfolio — Point72 has around $11 billion in assets — it's not bad for a day's work.

Update: This post has been editied to reflect the timing of Point72's purchase of the Celator position.