While he has played a character who was deaf on the show "NCIS", this does not necessarily mean he knows sign language in real life.
It is important to note that:
* Acting requires learning skills: Actors often learn new skills for their roles, including languages, sign language, musical instruments, etc.
* Personal life and acting roles are separate: Mark Harmon's personal life and his acting roles are distinct. While he may have learned some sign language for "NCIS", this doesn't automatically translate into real-life fluency.
Therefore, without official confirmation from Mark Harmon himself, it is impossible to know definitively whether he knows sign language.