A class action suit filed against Zenith National Insurance Corp. for allegedly discounting medical bills submitted for payment has no merit, according to Zenith CEO Stanley R. Zax. Zax said that the company had not yet been served a copy of the complaint, and that the company would fight it vigorously in court. He declined to comment further on the suit, filed in the United States District Court for the Central District of California by Kathleen Roche (doing business as Back Doctors Chiropractic) on her behalf and that of a purported class of similarly situated providers July 11. Neither Roche nor her lawyers responded to repeated messages left by the Business Journal for comment. Back Doctors’ Chiropractic is located in Belleville, Ill. A specialty workers’ compensation insurer, Zenith has headquarters in Woodland Hills. Nadra Kareem