Mad Men actress Elisabeth Moss has slammed former stage co-star Jeremy Piven, calling him "highly unprofessional".

Sitting on the couch for Bravo's Watch What Happens Live, 30-year-old Moss was asked about Piven.

The pair were co-stars in a play called Speed-the-Plow in 2008, which Piven left unexpectedly, citing mercury poisoning from eating too much fish in his diet.

Speaking to host Andy Cohen and fellow guest Jessica Walter, Moss explained that "it came out of nowhere. He just didn't come back one day."

Cohen had asked: "What besides eating too much sushi caused Jeremy Piven to leave Speed-the-Plow?", to which Moss replied: "That’s a good one, that’s a very good one, I could go on and on. Being highly unprofessional."

"(I've seen him) a couple of times, I saw him like a month later at the Golden Globes... When he was supposed to be really sick," Moss added sarcastically.