Steve Cohen disclosed a 2.8 million-share stake in a tiny pharmaceutical company that jumped 432% on Tuesday

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 some time in the past 10 days.

This position stood as of Monday's close.

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. Investors have a 10-day period from the time of purchase to disclose this position.

Based on Tuesday's trading, Cohen's stake increased in value by about $20.7 million from Monday's close to Tuesday's. Tuesday's filing doesn't say what Cohen's cost basis for the stake is.

Each share of Celator has been trading between about $1.50 and $2.10 over the last few weeks.

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.