Bill Ackman is buying up shares in a company that might manage your firm's payroll

Billionaire hedge fund manager Bill Ackman is loading up on shares of Advanced Data Processing, a New Jersey-based provider of payroll and benefits services.

The Pershing Square Capital Management CEO's initial purchases of ADP stock are already completed, but he may buy additional shares through new funds that he's launching, according to a Bloomberg News report.

It's not yet known how large Pershing's stake in ADP is, and it's unclear at this point whether Ackman will fight for changes in the company's operations, as he has in past investments.

ADP's stock spiked as much as 12% to $118.87 on the news. The shares had climbed just 3.1% year-to-date through Wednesday's close, less than one-third of the benchmark S&P 500's return.

This isn't the first time Ackman has owned ADP. Pershing was a shareholder in the company from 2009 to 2011 in a passive investment, according to Bloomberg data.

Beyond the company's human resource offerings, ADP's research institute is known for the monthly employment reports it publishes, which traders watch closely as a key measure of US economic strength.

In recent years, Ackman has perhaps been best known for his crusade against Herbalife, which he called a pyramid scheme. As an outspoken critic of the company, he publicly bet $1 billion against it, then took his share of lumps as the stock price failed to cooperate.