Ackman also told ADP that CEO Carlos Rodriguez "should be replaced". "However, ADP has a clearly defined Board nomination process, and the 2017 deadline for director nominations has been public for almost a year", the company said.
While Ackman may be able to score points with criticism of the company's strategy, he may find it hard to challenge Rodriguez given his track record as CEO, some analysts said.
In its statement, ADP said that its total return to shareholders under Mr. Rodriguez amounted to 202 percent, assuming investors held onto their shares of the CDK Global division that was spun off three years ago and reinvested the dividends from both companies.
"ADP has a clearly defined Board nomination process, and the 2017 deadline for director nominations has been public for almost a year", the company sniffed in a statement. ADP's board includes 10 people.
Pershing Square declined to comment for now.
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Ackman's pursuit of ADP, which has a market value of about $50-billion, comes as the 51-year-old hedge fund veteran is seeking to score a big win following disappointing investments in debt-laden drugmaker Valeant Pharmaceuticals International Inc and norovirus-stricken restaurant chain Chipotle Mexican Grill Inc. Pershing Square's investor return from 2012 through past year, according to ADP, was about 29 percent.
Pershing Square took a 9.9% stake in Chipotle previous year.
ADP defended Rodriguez in it statement and said investors have been rewarded with a total return of 202 percent - assuming they reinvested dividends and held on to shares of a unit that was spun off - during his almost six years at the helm. ADP said Ackman's fund owns an 8% stake, mostly made up of derivatives. Pershing Square's return over the same period is 29 percent, the statement claimed.
Shares of ADP are up 2.5% at $114.50 in premarket trading Friday.