The 47-year-old actor refused to engage after his former '24' co-star lashed out at him five years after they worked together.
Freddie, 38, recently claimed working on '24' in 2010 was a ''terrible'' experience and placed the blame solely on Kiefer, who is the star of the show.
Speaking about his decision to take a hiatus from Hollywood, he said: ''I did '24' ... it was terrible. I hated every moment of it. I just wanted to quit the business after that. So, I just sort of stopped.
''Kiefer was the most unprofessional dude in the world. That's not me talking trash, I'd say it to his face. I think everyone that's worked with him has said that.''
However, a spokesperson for Kiefer told Fox News: ''Kiefer worked with Freddie Prinze, Jr.more than five years ago, and this is the first he has heard of Freddie's grievances.
''Kiefer enjoyed working with Freddie and wishes him the best.''