Sept 6 Digital Generation Inc adopted a shareholder rights plan a month after it said it was considering a sale of the company.
The advertising distribution company rejected a takeover bid by rival Extreme Reach Inc, Reuters reported in June.
A shareholder rights plan, also called a poison pill, allows companies to issue new shares if an investor acquires shares over a certain threshold, diluting their holdings.
Digital Generation said the poison pill, in place until March 5 2013, will trigger if a shareholder acquires more than 10 percent of the company's shares.
Analysts have said that Digital Generation, formerly known as DG FastChannel, is a potential acquisition target for companies such as Google Inc and Yahoo Inc.