The plan has a 10 percent trigger, the same percentage stake that Icahn first reported on Thursday.
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In a U.S. Securities and Exchange Commission filing, Icahn said he may have discussions with the management of the steel maker and metals recycler regarding "business and strategic alternatives," believing the shares were "undervalued."
Commercial Metals' board in a statement said it adopted the plan following Icahn's "sudden and rapid ownership increase," and an indication from an Icahn representative of his "intention to continue accumulating CMC stock."
The company said it plans to meet with Icahn in September to better understand his interest.
Stockholder rights plans are commonly known as "poison pills," and can dilute the power of activist shareholders.
Under Commercial Metals' plan, if the rights become exercisable, shareholders can buy a fraction of a share in Commercial Metals which will have economic and voting terms similar to those of one share of common stock.
Commercial Metals shares closed up 21 cents on Friday at $14.51.
(By Jonathan Stempel and Megan Davies; editing by Bernard Orr)