For the fairer half of the population, the Marc Jacobs bag is an essential accoutrement on par with a house key or driver's license: Leaving home without all three can cause a nagging lack of mobility, social or otherwise. Now, with the introduction of his limited edition gym bag, the Midas-touch designer has loosed these coveted wares on men as well. Conceived as a carrier for cross trainers or Babolat racquets, it sports a silver padlock and wouldn't look out of place on a locker room bench, next to some sadder sacks. But with its smooth suede lining and understated blue, baby-blue, or white leather exterior, duly devoid of logos or gold accents, it could moonlight as your carry-on when a briefcase might seem a tad too solemn for the occasion. A scant 100 of the bags have been manufactured — handcrafted in Spain of Italian and Spanish calfskin — and are available only at Mr. Jacobs's eponymous boutiques. Which means that for a lucky few, the shame of a tailored suit mismatched with smelly Gore-Tex duffel will be no more.