Over the years a number of famous (or soon to be famous) actors have appeared in M*A*S*H.
Note: Actors that have recurring roles (e.g. Allan Arbus) can be found in the Other Characters section.

Robert Alda
Character: Dr. Anthony Borelli
Episodes: 'The Consultant', 'Lend A Hand'
The father of Alan Alda. He was a very popular actor in the forties and fifties with movies like The Beast with Five Fingers.
Ned Beatty
Character: Col. Hollister
Episode: Dear Peggy
Starred in 'Homocide: Life On The Streets'
Billy Green Bush
Character: The Cowboy (John Hodges)
Episode: Cowboy
Andrew Clay
Character: Corp. Hrabosky
Episode: Trick or Treatment
James Cromwell
Character: Leo Bardanaro
Episode: Last Laugh
Brian Dennehy
Character: M.P
Episode: 'Souvenirs'
Larry Fishburn
Character: Corp.Mathew Dorsey
Episode: 'The Tooth Shall Set You Free'
Charles Hallahan
Character:
Episode: 'Taking the Fifth'
Mariette Hartley
Character: Inga Halvorsen
Episode: 'Inga'
Pat Hingle
Character: Col. Daniel Webster Tucker
Episode: 'April Fools'
Peter Jurasik
Character:
Episode: 'Snap Judgement/Snappier Judgement'
Played Londo Mollari in Babylon 5
Linda Kelsey
Character: Nurse Baker
Episode: 'The Nurses'
Starred in Lou Grant
Shelley Long
Character: Nurse Mendenhal
Episode: 'Bottle Fatigue'
Played Diane Chambers in Cheers
Presented Memories of M*A*S*H
Mako
Character:
Episode: 'Rainbow Bridge'
Leslie Nielsen
Character: Col Buzz Brighton
Episode: 'The Ringbanger'
Originally a serious actor, he played Frank Drebin in the series Police Squad (and later the 'Naked Gun' movies)
Stafford Repp
Character: Sgt. Clay
Episode: 'White Gold'
Susan St. James
Character: Aggie O'Shea
Episode: War Co-Respondent
Patrick Swayze
Character: Gary Sturgis
Episode: Blood Brothers
Hit the big time with the movies Dirty Dancing and Ghost
George Wendt
Character:
Episode: Trick or Treatment
Played Norm in the hit series Cheers
Larry Wilcox
Character: Mulligan
Episode: The General's Practitioner
Starred in the series Chips
George Wyner
Character: Benson
Episode: Potter's Retirement
Joan Van Ark
Character: Lieutenant Erica Johnson
Episode: Radar's Report
Played Val Ewing in the soap Knotts Landing
Sorrell Booke
Character: General Barker
Episode: Requiem for a Lightweight, Chief Surgeon Who?
Played Boss Jefferson Davis Hogg in The Dukes of Hazzard
Beeson Carroll
Character: Donald Penobscott
Episode: Margaret's Marriage
Barry Corbin
Character:
Episode: Your Retention Please
Blythe Danner
Character: Carlye Breslin
Episode: 'The More I See You'
Dennis Dugan
Character: Danny McShane/
Episode: 'Love and Marriage'/'Strange Bedfellows'
Teri Garr
Character: Lt. Marquette
Episode: 'Sniper'
Gregory Harrison
Character: Baker
Episode: 'The Nurses'
Played Gonzo Gates in Trapper John, M.D.
Edward Herrman
Character: Capt. Steve Newsome
Episode: 'Heal Thyself'
Ron Howard
Character: Pte. Wendell Peters
Episode: 'Sometimes You Hear the Bullet'
Probably best known for playing Richie Cunningham in Happy Day's,
Ron Howard is now a very successful movie director.
Alex Karras
Character: Lyle Wesson
Episode: 'Springtime'
Bruno Kirby
Character: Boone
Episode: Pilot
Keye Luke
Character:
Episode: 'Death Takes a Holiday'
Stuart Margolin
Character: Mandel Sherman / Stanley Robbins
Episodes: 'Bananas, Crackers, and Nuts' 'Operation Noselift'
Nicholas Pryor
Character: Major Nathaniel Burnham
Episode: 'Rumor at the Top'
John Ritter
Character: Pte. Carter
Episode: Deal Me Out
James B. Sikking
Character: Finance officer
Episode: Tuttle
Played Lieutenant Hunter in the classic t.v series Hill Street Blues and also Dougie Howser's dad.
Jeffrey Tambor
Character: Reddish
Episode: Foreign Affairs
Mary Wickes
Character: Col. Reese
Episode: 'House Arrest'
Rita Wilson
Character: Nurse Lacey
Episode: Blood And Guts
She's married to the actor Tom Hanks
Burt Young
Character:
Episode: L.I.P