Icahn has targeted inversions, when a company changes its domicile to take advantage of lower tax rates elsewhere. The billionaire investor, who recently announced he would put $150 million into a super PAC to push for tax changes, said his warnings that companies could depart were coming true.
Icahn contends companies keep money overseas or leave the United States altogether to avoid "double taxation," called repatriation. Icahn implied that pharmaceutical giant Pfizer plans to move out of the country, though the company did not say that in a statement earlier in the day.
"Pfizer emphasizes that no agreement has been reached and there can be no certainty that these discussions will lead to a transaction, or as to the terms on which a transaction, if any, might be agreed," the company said.
Icahn once again called for new international tax provisions in a highway bill before Congress to help keep companies within the United States. He does have a possible interest in changes as one of Apple's largest shareholders — the company has nearly $200 billion in cash parked overseas.
Pfizer did not immediately respond to a CNBC request to comment.