Combine sugar, margarine, corn syrup and salt in a microwave pan and bring mixture to boil in a microwave oven set on "high."
Boil for 3 minutes.
Remove from oven and stir in baking soda, mixing well.
Pour over popcorn.
Mix to coat well.
Pour caramel corn into a large brown grocery bag.
Fold the top of the bag down loosely until it will fit in the microwave oven.
Cook on the highest setting for 3 minutes; shake sack well every minute during cooking.
Cool the caramel corn on waxed paper.
Store in a 5-quart plastic ice cream pail or in other suitable containers.