TRENTON, N.J. — Johnson & Johnson, making a big jump into the risky but potentially lucrative field of Alzheimer's disease, is taking a major stake in Irish biopharmaceutical company Elan Corp., investing up to $1.5 billion initially.

The two will cooperate to complete research on two injected drugs to stop progression of the mind-robbing disease and on a vaccine to prevent the buildup of plaque in the brain that causes increasing memory loss, confusion, wandering and aggression.

The deal, announced Thursday, sent Elan's U.S. shares up 60 cents, or 8.6 percent, to close at $7.60. J&J shares fell $1.09, or 1.9 percent, to $55.98.

The agreement would make J&J a major player in one of the biggest areas of unmet medical need and bring any successful treatments to market faster.

"It's good news for Alzheimer's patients and their families," said Erik Gordon, an analyst and professor at the University of Michigan's Ross School of Business.

New Brunswick, N.J.-based J&J, the world's biggest health care company with nearly $64 billion in 2008 sales, can pour far more money than Dublin-based Elan into the research, he said. J&J sells one Alzheimer's drug, Razadyne, with sales of just $541 million last year, but company executives last month said Alzheimer's disease is now a priority.

Several drugs help limit memory loss and other symptoms, but there are no treatments to stop or reverse brain damage caused by the disease. The market is huge: roughly 5.3 million current U.S. Alzheimer's patients, and more than 100 million worldwide expected eventually as the population ages.

J&J plans to commit up to $500 million initially for development of bapineuzumab, the compound furthest along in testing .

J&J will get Elan's 50 percent stake in its Alzheimer's immunotherapy research partnership with Madison, N.J.-based Wyeth, and set up a new joint company that will be 49.9 percent owned by Elan. J&J will gain all rights to any drugs approved; Elan will get 49.9 percent of the profits, plus royalties on sales.