Like a few other devilish whitehaired and fabulously venerable old men, James Cagney, 79, now bears a noticeable resemblance to the Wizard of Oz. Sitting in a very easy chair in his Dutchess County, N.Y., home, Cagney says, "I've got what I call an Irish memory," and this turns out to be much more of a blessing than a curse.

The beneficiaries are the producers and viewers of ABC's "Good Morning America" which, starting this morning and continuing through Friday, will broadcast in four segments the first full-scale TV interview Cagney has given in 30 years. The last time he did it, even his interviewer, Jack Paar, was a pup.

This time the interviewer is considerably less formidable and resourceful -- the star-struck, awestruck and just plain stricken David Hartman, host of the program. Hartman lacks the competence to get an essentially reticent figure like Cagney to elaborate on terse answers.

But any chance to see Cagney now is automatically a privilege. On the first segment, he talks about his youth and his mother. "She was a character," he says, who had the "guts of any army," and kept him from what might have been a life of crime. "Are you a tough guy?" Hartman asks. "As far as I know, I'm not," Cagney says.

The Wednesday and Thursday segments deal with Cagney's show biz career. "I used to throw up before every show," during his vaudeville days, Cagney says, and he was shocked that his first movie role led to others because he thought he was "nothing to photograph."

On Thursday, Cagney talks about his most indelible screen portrayal, George M. Cohan in "Yankee Doodle Dandy," and the trademark tightening of his shoulder that, though he used it in only one film, Cagney impersonators never fail to incorporate into their acts. On Friday he discusses his 56-year marriage and three times emphasizes that his goal in life is to have "no strain."

"I have no fixed convictions about anything, really," James Cagney says.

Intercut throughout the interview sequences are film clips and comments from such Cagney pals and fans as George Raft, Pat O'Brien, Ralph Bellamy and Jack Lemmon. They appear in montage at the very end to say how dear the man is to their hearts. It hardly seems a minority view.

In Washington, "Good Morning America" is seen from 7 to 9 a.m. on Channel 7. The first Cagney segment will air during the last half hour of today's show, the second between 7:30 and 8 Wednesday, the third between 8:30 and 9 Thursday, the last between 7:30 and 8 Friday.

Executive producer George Merlis said yesterday that the Cagney interview was taped in about an hour at Cagney's home in December. The interview was saved for now, Merlis said, because February is a ratings "sweep" month and the time to play one's top cards.

He was considered an "ungettable" interviewee, Merlis said, but now that there are tentative plans for a movie version of his life, Cagney decided he would do one TV show. "Having occasionally seen our show and because he likes David Hartman, he chose us," said Merlis. "He felt that in doing the interview with us there would be no strain." A little strain might have helped, but the Cagney interviews are still imensely enjoyable television.