MICROWAVE peanut butter and margarine in large microwaveable bowl on MEDIUM 1 min. or until margarine is melted. Stir until mixture is well blended.

ADD marshmallows; toss to coat. Microwave an additional 1-1/2 min. or until marshmallows are puffed; stir until well blended. Add cereal; mix well.

PRESS mixture firmly into greased 9-inch square pan. Let stand 1 hour or until set before cutting into 20 bars to serve.