Three shareholders owning 12.3% of Chiron's ( CHIR) stock say they oppose an offer by Novartis ( NVS) to buy the shares that the Swiss company doesn't already own. The announcement heightens the uncertainty of Novartis' takeover bid, which is worth $45 a share, or $5.1 billion. Novartis now owns 44% of Chiron. News of the holdouts sent Chiron's stock to a 52-week high of $45.32, up 61 cents, or 1.4% for the day, on nearly twice the average daily volume. After hours, the stock slipped 3 cents. The new opponents join ValueAct Capital , a San Francisco investment management firm, in vowing to vote against the deal. ValueAct owns 5.2% of Chiron's shares. One of the dissidents, CAM North America LLC, announced the newest opposition in a filing with the Securities and Exchange Commission. New York-based CAM and two other investment advisory firms now resisting the bid -- Smith Barney Fund Management LLC and Salomon Brothers Asset Management Inc. -- hold about 23.2 million shares. In order to complete the deal, Novartis needs approval by a majority of Chiron shareholders, excluding the stock controlled by Novartis. ValueAct owns about 9.8 million of Emeryville, Calif.-based Chiron's shares. Together, the four opponent firms own 17.5% of all Chiron shares. More importantly, their holdings represent about 30% of the shares eligible to vote. "We believe the merger consideration offered by Novartis of $45 in cash per Chiron share is financially inadequate," says Richard A. Freeman, managing director of CAM North America, in a Jan. 25 letter to Chiron that was enclosed in the SEC filing. Although Novartis raised its buyout bid to $45 a share in October after Chiron's board rejected an earlier $40-a-share proposal, Novartis says it won't pay more . Novartis already has announced how the various Chiron units -- vaccines, blood testing and biopharmaceuticals -- will be absorbed. The deal has been approved by the Federal Trade Commission. It still requires clearance from the European Union and Chiron's shareholders. A shareholders' rebellion creates a dilemma for both companies. Chiron has already signaled it would be better off being owned by someone else rather than being independent. If the shareholders say "no" and Novartis won't budge on price, the question would be: Could Chiron attract another buyer, knowing that Novartis owns a giant piece of the company?