Roche is seeking to acquire Tucson, Arizona-based Ventana, which makes diagnostic tests, for $75 a share. Ventana had wanted to apply an Arizona law that makes it harder for companies to be bought, but the judge ruled that it could not use that law, Roche said.

In a separate case, Roche is also contesting a so-called poison pill defense by Ventana, which would give Ventana shareholders rights to buy new shares at half price if Roche acquires 20 percent of the company.

Roche said on Tuesday it extended by four weeks the expiry of its tender offer to September 20 from August 23.