John Wagner paraded freshly groomed young horses around a sale ring at Rosecroft Raceway's third annual harness horse auction yesterday while his father, Earl, and mother, Wilma, kept a keen eye on the animals from the audience, checking for ankle knots or other flaws that might make the horse a poor risk.

When the younger Wagner, who usually drives the horses, wanted to take a break from leading the sale horses through the ring, his wife, Robin, took over..

It was not unusual to find husband-and-wife teams, and even whole families, such as the Wagners of Glenn Dale, at the auction yesterday. For harness racing, the horse sport long overshadowed in Maryland by the more glamorous and more lucrative thoroughbred racing, has traditionally been a family affair.

"I've been messin' around with horses since about 1946," said Earl Wagner, who dutifully recorded the price paid for each horse in the white booklet handed out to the prospective buyers. The book gave a detailed description of each horse's blood lines and the amount of money its ancestors have won on the track.

Wagner, his wife explained, used to work as a salesman for the Continental Baking Co. and instead of coming home from the office, would go straight to the stables at Rosecroft in Fort Washington to clean and jog his horses. Wagner said he caught "horse fever" from his father-in-law, who raised livestock and also kept harness horses.

Unlike thoroughbreds, which are bred to run, harness horses are bred to trot and pace. When they race, they are driven by horsemen who guide them from a sulky, a two-wheeled cart pulled by the horse.

Some people at yesterday's auction expressed concern that the growing popularity of harness racing in Maryland and efforts within the industry to increase the purses--the award money paid to winning horses--will soon drive up the price of the horses. That, in turn, would make it more difficult for people such as the Wagners to participate--people who say they are in it not so much for the big money, but because "it gets in your blood."

"I'm terribly afraid harness racing will go the way of thoroughbred racing, where you have these great high-priced horses and a limited number of people who can afford to be in the sport," said Hazell Smith, a horse breeder from Lewes, Del., who put six yearlings and a 3-year-old in the Rosecroft sale.

Changes that have come to Rosecroft in recent years reflect the changes that are already overtaking the harness racing industry. Only in the last four years, since 1980, has Rosecroft had a $100,000 purse for a single race. Next year, Rosecroft expects to have at least one race that goes over that mark.

This has been the first year that harness racing will go on year-round in Maryland, instead of ending in mid-September. Rosecroft will share the additional racing days with Freestate Raceway in Laurel, the only other harness track in the state.

Yesterday, most of the horses sold for under $2,000. The highest bid was $11,500 for a yearling, a horse born in 1982. That was far short of the $42,000 record price for the Rosecroft auction.

"Harness racing has always been a family thing. It's been mostly farmers in it who know the blood lines of the horses," Smith said. "You could go into the stables and you'd see all the kids in the family down around the legs of the horse. This is what we stand to lose."

Indeed the whole family of horse racing--owners, breeders, trainers, drivers, judges--came together yesterday at the auction in a world distinctly different from the shopping malls and suburban residences that surround Rosecroft.

The uniform of the day was a brightly colored baseball cap, plaid flannel shirt and dust-covered cowboy boots. It was a world that has its own language--brood mare (female horse used principally for breeding), foal (newborn horse), sire (father of horses), and weanling (horse newly weaned from its mother).

"I'm a 15, put 'em a 16," auctioneer Billy Perkins rattled off without a breath. That means, "I've got a bid of $1,500, will anyone bid $1,600?" explained Perkins, director of racing at Rosecroft who attended auctioneer's school to learn to talk like that.