Grease 1 baking sheet.
In heavy 3-quart saucepan stir together corn syrup, sugar, water and margarine.
Cook over medium heat, stirring constantly, until sugar is dissolved and mixture comes to boil.
Continue cooking without stirring until temperature reaches 280°F. on candy thermometer, or until small amount dropped into very cold water separates into threads which are hard but not brittle.
Gradually stir in peanuts; continue cooking, stirring frequently, until temperature reaches 300°F., or until small amount of mixture dropped into very cold water separates into threads which are hard and brittle.
Remove from heat; stir in baking soda.
Immediately pour mixture onto baking sheet.
Spread mixture evenly to edges with greased metal spatula.
Cool.
Break into pieces.