Horses used on the television series Luck were drugged, underweight and sick during production, a lawsuit claims.
The high-profile show focused on the seedy side of horse-racing in the US, and was made by HBO.
It starred Oscar-wining Dustin Hoffman, featured British actors Michael Gambon and Ian Hart, and was shown on Sky Atlantic last year. But it was cancelled last March during the filming of a second series after several horses were killed.
Animal rights worker Barbara Casey claims she was wrongly sacked from her post at the American Humane Association after complaining about the treatment of horses on the show.
Her lawsuit says four horses died during the show's production, not three as previously reported.
It alleges horses were "often drugged to perform" and "underweight and sick horses unsuited for work were routinely used" by producers.
She is suing HBO, which has repeatedly denied abusing horses on the show, and the humane association, where she was director of the film and television unit, which oversees animal welfare.
The lawsuit says she urged the association to report HBO and producers to authorities for possible animal cruelty criminal charges.
But it "bowed to political and financial pressure and refused to report the production defendants' conduct to the authorities", it states.
HBO said in a statement: "We took every precaution to ensure that our horses were treated humanely and with the utmost care, exceeding every safeguard of all protocols and guidelines required of the production."