The premiere episode of Alex Haley's "Roots" on ABC Sunday night attracted what the network is calling the six-largest TV audience ever, a total of 28.8 million households or roughly 75 million viewers.

Overnight Nielsen rating in New York, Los Angeles and Chicago for Monday night's second "Roots" episode also indicate that the eight-part saga of an American black family was outdrawing programming on CBS and NBC in those cities by more than 2 to 1.

ABC was not surprised by the showing and national advertisers had long long ago bought out every available minute for the 12 hours of "Roots" at a rate 'well over $100,000 a minutes," according to a network spokesman.

Meanwhile, CBS and NBC apparently decided to stay with their recently-revamped regular nighttime schedules for the most part during the "Roots" week.

CBS "Helter Skelter" two-part re-run was schedules back in December, while NBC counter-Programmed only two surprises for the week, a special on John F. Kennedy Thursday night and the introduction of "Lanigan's Rabbi", a new Sunday mystery entry starring Art Carney.

An ABC spokesman said yesterday that only Parts One and Two of "Gone With the Wind" on NBC last November and the last three football Super Bowls outdrew Sunday night's "Roots" audience.

"Roots" Part One's showing also helped ABC win the 18th week of the TV season in the Nielsen rankings, as the top six programs for the week of Jan. 17-23 were from that network.

Other top-10 finishers were, in order."Laverne and shirley," "Happy Days," "$6 Million Man," "Charlie's Angels," "Baretta," Bob Hope's Friday night special, "M*A*S*H*," "Man in th Iron-Mask" and "One Day at a Time.

Incidentally, an ABC News special on the Equal Rights Amendment finished 68th among the 68 prime-time programs ranked last week by Nielsen.

Channel 7 officials are considering the call letters WJLA among its top choices for the new station identification that must be submitted soon to Federal Communications Commission.

WJLA just happens to contain the initials of Channel 7 owner Joe L. Albritton, who under an FCC ruling must sell his interest in either The Washington Evening Star or Channel 7 by January 1979.

WHen Washington Star Communications Inc., the parent company for both The Star and the TV station, sold WMAL-AM and WMAL-FM to ABC in December for $16 million, the network purchased the familiar call letters, necessitating the revision. It is due to go into effect this summer.

The FCC recently gave WMAL-TV a waiver on its deadline for submitting new call letters, but the sation hopes to make a decision on the matter before the end of the month.