Novartis ( NVS) said Wednesday that it won't raise its $45-a-share price for a majority of Chiron ( CHIR), the Emeryville, Calif., maker of vaccines, drugs and blood-testing products.

The Swiss drug company was responding to a letter that ValueAct Capital, an investment management firm in San Francisco, sent to Chiron, saying the offer was too low. ValueAct, the owner of about 5% of Chiron's stock, says it will vote against the Novartis offer.

Novartis is seeking to buy the 56% of Chiron's shares that it doesn't own. The company made a $40-a-share bid in September, but Chiron's board rejected the proposal. In late October, Novartis raised the price.

The Federal Trade Commission has approved the transaction. The companies need clearance from the European Union and a majority of Chiron shares, excluding those owned by Novartis.

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