The most popular, most tour-validated and most revolutionary ball in the history of golf is now the principal exhibit in a lawsuit involving golf's two largest companies. On Thursday, Callaway Golf filed a complaint in U.S. District Court in Delaware against Acushnet, alleging that Acushnet's Titleist Pro V1 line of golf balls infringes on four or more golf ball patents owned by Callaway. The lawsuit, says Callaway spokesman Larry Dorman, was filed "only after repeated attempts to negotiate a settlement failed." He would not specify how long the companies were negotiating, other than to say "quite a while." According to multiple sources familiar with the situation, the two companies had been involved in discussions over golf-ball intellectual property since at least last summer. "We are reviewing the lawsuit and have no comment on it at this time," said Joe Gomes, Titleist spokesman. The intellectual property in question, referred to in the complaint as "the Sullivan patents," focuses primarily on the construction of a multilayer ball with a solid core and a polyurethane cover. The patents were acquired by Callaway in 2003 when it purchased Top-Flite Golf from bankruptcy.