Bill Ackman's Pershing Square Capital Management owns 2 percent of Automatic Data Processing common shares after converting only part of a larger derivative position he has taken on this year as he wages a proxy fight.
CNBC's David Faber reported Monday that Ackman still holds the rest of his overall 8.3 percent stake in the payroll processing giant in derivatives, adding that people familiar with the matter believe he didn't want to spend the money converting all of the position to common shares.
ADP shares were down 0.15 percent on Monday morning, to $108.25, but they are up 5.29 percent for the year so far. Last month ADP disclosed it had been approached by Ackman, and the two sides have engaged in a war of words for most of the last month.
Ackman is trying to get three seats on ADP's board for himself and two others and has outlined a plan to boost the company's performance. ADP has rejected his board nominees.
Last week the two sides met and Ackman offered to drop the proxy contest if he and the two nominees were added to the board. But ADP, in a statement Thursday, said it would not do that and called Pershing's plan "vague" and "risky" and something that "could do serious harm."