In 3-quart microproof casserole, melt butter on HI (max. power) 1 minute.
Add marshmallows and cook, covered, on HI (max. power) 2 to 2 1/2 minutes, or until soft, stirring once.
Stir in peanut butter until smooth.
Mix in cereal and peanuts.
Press warm mixture into lightly buttered 12 x 7 x 2-inch dish.
Cool and cut into bars.