ST. PETERSBURG, Fla. » New York Yankees legend Mickey Mantle signed a baseball for Dennis Schrader in 1956, when Schrader was a 9-year-old boy attending a spring training game in Florida, leading to a lifelong obsession: He now has more than 4,600 signed baseballs, certified by Guinness as the largest such collection in the world.

That obsession is now on display at the St. Petersburg Museum of History in Florida. "Schrader’s Little Cooperstown" opened last week, and Schrader was grinning from ear to ear. He and his wife have loaned the balls to the museum for 20 years, after which they will be returned to the family.

Previously, Schrader’s baseballs were displayed in a 12-by-14-foot room in his home that had walls a foot thick, a bank vault door, motion sensors and video camera surveillance. The semiretired mobile home executive once spent $25,000 on a single ball, signed by Joe DiMaggio and then-wife Marilyn Monroe.

He estimates the collection is worth $2 million to $3 million.

The collection is a trip through baseball history, and Schrader will personally give tours of the collection to groups.

There are the obvious great signatures: Mickey Mantle, Babe Ruth and Jackie Robinson. There are several Negro League balls, a tribute to the All-American Girls Professional Baseball League featured in the movie "A League of Their Own," and several signed by celebrities and politicians.

"He captured the essence of baseball," said St. Petersburg Mayor Bill Foster.

In August 2011, Guinness World Records certified Schrader as the owner of 4,020 baseballs signed by Major League Baseball players. Duplicates and balls signed by nonbaseball celebrities — including President Barack Obama — brought his collection to more than 4,600.

It cost the museum $300,000 to design the exhibit and two years for city officials to persuade Schrader to loan the precious collection.

The museum, which sits on St. Petersburg’s downtown waterfront, is also gearing up to celebrate the 100th anniversary of baseball spring training in the city. Spring training began in St. Petersburg in 1914 with the St. Louis Browns playing at Coffee Pot Park.

Schrader admitted that "there’s an emptiness" in his home without the baseballs but said the vault contains other collectibles, including his wife’s 500 cookie jars and several hundred celebrity-autographed photos.

Schrader’s wife, Mary, said she and her husband won’t stop collecting signed baseballs.

"In fact, I have a ball in my purse right now," Mary Schrader said, laughing and showing the blank ball. "I always carry one around, because you never know who you’ll run into."