W. A. (Tony) Boyle was making his way feebly across the stone plaza and up the steps of the Delaware County Courthouse here on the opening day of his second murder trial when a camera crew from a Philadelphia television station caught up with him.

As the camera whirred, the reporter leaned with her microphone toward the infirm, 76-year-old former president of the United Mine Workers union who was being propped up by lawyers at each arm, and asked him, "What do you think about this trial, Mr. Yablonski?"

Indeed, it has been a long time - nearly four years - since Boyle was convicted and sentenced to three life terms for ordering the murders of his union rival, Joseph (Jock) Yablonski, 59; Yablonski's wife, Margaret, 57; and their daughter, Charlotte, 25. The three were shot to death as they slept in their rural Clarksville, Pa. home on Dec. 31, 1969, three weeks after Yablonski was defeated overwhelmingly by Boyle in a UMW election.

(After the murders, the election results were overturned to the Labor Department for vote-buying and election fraud. In a new election, Boyle was defeated by Arnold Miller, a Yablonski partisan.)

A year ago, the Pennsylvania Supreme Court set aside the verdict and ordered a new trial, saying that Delaware County Judge Francis J. Cafania had erred when he refused to allow certain defense witnesses to testify.

Now, after a four-month delay because of Boyle's heart condition, two snowstorms and a minor fire in Boyle's motel late last week that prompted his attorney to take him to a hospital for treatment of smoke inhalation, the trial will get under way before a jury today.

Most of the proceedings inspire a feeling of deja vu for anyone who followed the closely watched first trial in 1974. It is being held in the same pink, neo-classic courtroom 15 miles west of Philadelphia. It is being held before the same judge. And, of course, it is being conducted by the same special prosecutor, Richard A. Sprague.

But there is a new wrinkle in this case.

Sprague, who established a national reputation for his pursuit of high-ranking UMW officials responsible for arranging the murders, is also on trial, thanks to Boyle's new lawyer, A. Charles Peruto, who is trying hard to tarnish Sprague's "Mr. Clean" image here by implying that Sprague made questionable deals with participants in the murders in order to obtain Boyle's conviction.

In his opening statement, Peruto contended that Boyle was implicated by other UMW officials who had nothing to lose and everything to gain by cooperating with Sprague.

He told the jury on Saturday that the Yablonski conspirators decided the best strategy was to "fight the case. If you lose, go to Mr. Sprague. I can't blame him. He was furthering his own career."

To prosecute Boyle again, Sprague interrupted his private Phildelphia law practice, to which he returned after serving a turbulent, five-month stint in Washington as chief counsel of the House Select Committee on Asassinations. He resigned that post last March in an angry dispute with the committee chairman on strategy and cost of the assassinations probes.

Here is a brief rundown of the key participants:

Paul Gilly, 45, a house painter from Cleveland who recruited Aubran Martin, 28, and Claude Vealey to kill Yablonski for $1,750 each. Vealey, 34, confessed in June 1971, giving details of the plot and implicating the other two. Martin and Gilly are serving life prison terms and Vealey also is in prison.

Annette Gilly, wife of Paul Gilly, confessed in April 1972, that she was the liaison between her husband and her father, Silous Huddleston, former president UMW local in LaFollette, Tenn., who hired the killers. She said cash came from higher union leaders. Annette Gilly, 39, and her father, both were given new identities and relocated by federal officials.

William J. Prater, 56, a UMW field representative from LaFollette, was convicted of murder of his part in the conspiracy and sentenced to life in prison. He helped raise the money to pay the killers.

Albert Pass, 55, secretary-treasurer of the UMW's District 19, was convicted of murder and is serving a life sentence. Huddleston named Prater and Pass as the men who passed him the murder payoff. Peruto will contend in this trial that it was Pass, not Boyle, who ordered Yablonski's murder.

William Turnblazer, 55, was a lawyer and president of District 19. He said he witnessed Boyle order Pass to kill Yablonski. He is serving a 15-year federal term for violating Yablonski's civil rights.