All-America nose guard Tony Casillas of Oklahoma announced yesterday that he has signed with the Atlanta Falcons, who chose him in the first round of the National Football League draft.

Casillas, the second player selected in the NFL draft after Heisman Trophy winner Bo Jackson of Auburn, said the agreement covered four years. He would not disclose the amount, but said it was in the vicinity of the offer made to him by the Arizona Outlaws of the U.S. Football League.

The Atlanta Journal reported the four-year deal is valued at $2.35 million, including a $1.43 million signing bonus, a $1 million loan and annual salaries of $175,000, $200,000, $250,000 and $300,000.

The Outlaws had offered Casillas contracts ranging from one to four years in length, with the four-year deal reportedly worth $2.5 million.

Cardinals: Running back Stump Mitchell, who had threatened to sign with the USFL Arizona Outlaws, signed a contract with St. Louis and headed to the NFL team's training camp in Charleston, Ill.

Mitchell was in Phoenix Monday prepared to sign with the Outlaws when he changed his mind and agreed to a three-year contract worth $1.2 million during a telephone conversation with Cardinals attorney Bob Wallace.

Wallace met Mitchell and his agent, Harold Lewis, at the airport on their return to St. Louis and the contract was signed.

Lewis said his client was in the offices of Outlaws president Bill Tatham Jr. poised to sign a three-year contract for $1.5 million.

"We were all sitting there, and the contract was in order, and it came time to sign and I looked at Stump, and he didn't seem happy," Lewis said.

The agent said he excused himself and phoned Wallace, who called Cardinals owner Bill Bidwill. The owner improved St. Louis' offer, and a deal was struck.

"I'm glad that Harold made that last call," Mitchell said. "I was awfully close to signing that USFL contract . . .

"I'm glad to be staying with the Cardinals, because that's where my heart is, and I never wanted to leave. Tell the guys at training camp that I'm on my way."

Colts: Fearful of pounding on a still-shaky knee, Indianapolis nose tackle Leo Wisniewski says he is retiring. Wisniewski, 26, a second-round draft pick out of Penn State in 1982, underwent knee surgery before the 1985 season and sat out the entire year.

"I really had planned to play another year," he said. "In May when I was tested by the Colts, my strength was good and my weight was back above 250. But I was never sure of the knee. I was experiencing some pain during the workouts, and I just didn't know if I was ready for the giant headache of a training camp where I needed eight to 10 aspirins a day."

Bears: Defensive end Richard Dent brightened the opening of Super Bowl champion Chicago's training camp in Platteville, Wis., by agreeing to report later in the week.

Dent and first-round draft pick Neal Anderson were not expected to be among the roughly 50 veterans and 40 newcomers who were to meet last night with Coach Mike Ditka and his coaching staff.