LOS ANGELES - Robert Downey Jr. is trading Iron Man's metal suit for Sherlock Holmes' deerstalker hat. Downey is following his action blockbuster "Iron Man" with the title role in "Sherlock Holmes," a mystery drama based on Arthur Conan Doyle's detective and an upcoming comic-book about Holmes by Lionel Wigram, one of the movie's producers.

Guy Ritchie ("Lock, Stock and Two Smoking Barrels") is directing the Warner Bros. film, which is set to begin shooting in October, the studio said Thursday.

That should put it ahead of the schedule for a "Sherlock Holmes" comedy in development at Sony, which will star Sacha Baron Cohen as the detective and Will Ferrell as his partner, Dr. Watson.

No other cast members have been announced for Downey's film, which will be set in Holmes' late 19th-century London. The producers include Joel Silver and his producing partner - Downey's wife, Susan Downey - who previously worked with the actor on "Gothika" and "Kiss Kiss Bang Bang."

Downey co-stars with Ben Stiller and Jack Black in the August comedy "Tropic Thunder," about a group of pampered actors stranded among dangerous drug-dealers while shooting a Vietnam War movie in the Asian jungles. This fall, Downey stars with Jamie Foxx in "The Soloist," a drama about a schizophrenic music prodigy living on Los Angeles' skid row.