Preheat oven to 325 degrees F (165 degrees C). Lightly grease a 5x9 inch loaf pan.
In a large mixing bowl, cream together butter and sugar. Add eggs; beat well. Stir in peanut butter, bananas, flour and baking soda until blended. Fold in walnuts. Pour into prepared pan.
Bake at 325 degrees F (165 degrees C) for 70 minutes, or until a toothpick inserted into center of the loaf comes out clean. Remove to a wire rack to cool.