Safeway announced Tuesday its adoption of a shareholders rights plan, or “poison pill,” that will cap any stake in the grocery giant at 10 percent to ward off unwanted advances.

The move came on the heels of a disclosure by Jana Partners—a $6 billion hedge fund headed by Barry Rosenstein—in a regulatory filing that it had accumulated a 6.2 percent stake in Safeway at an aggregate purchase price of nearly $320 million.