Scrub potatoes under running water. Cut potatoes into 1-inch cubes. Place potatoes in a medium saucepan; add salted water to cover. Bring to boiling; reduce heat. Simmer, covered, for 20 to 25 minutes or until just tender. Drain well.
Meanwhile, combine mayonnaise or salad dressing, red pepper, celery, lemon juice, mustard, salt, and black pepper in a large mixing bowl. Gently stir in potatoes until evenly coated with dressing. Cover for 2 hours or until serving. Makes 6 servings.