Preheat oven to 350°F. Lightly grease 11- x 7-inch baking dish.
Mix together cooked chicken, cream of chicken soup, poppy seeds, and sour cream. Spoon mixture into dish.
Pour melted butter over cracker crumbs and stir together. Sprinkle over casserole.
Bake for 35 minutes or until hot and bubbly.
