While Jerry appeared in all 180 episodes, the other 3 major characters were not always present:
George appears in every episode except for "The Pen" (third season). In the DVD extras, Jason Alexander confessed that he told Larry David he would quit if he was ever written out of the story line again.
Kramer appeared in all but two episodes: "The Chinese Restaurant" and "The Pen", in the second and third seasons, respectively.
Elaine did not appear in three total episodes: The first episode, The Trip (Part 1) and The Trip (Part 2). Also, Elaine appears only minimally in "The Pitch" and "The Ticket", due to maternity leave.