Peanut Butter Popcorn Balls

4 quarts popped corn
1/2 cup light corn syrup
1/2 cup creamy peanut butter
1/2 cup granulated sugar
1 teaspoon cream of tartar
1/4 teaspoon baking soda
2 tablespoons margarine

Place corn syrup, sugar and cream of tartar in large saucepan. Bring to a boil; boil until softball stage on candy thermometer. Remove from heat and stir in peanut butter. Add baking soda and margarine. Stir until melted and pour immediately over popcorn in large bowl. Form into balls.