On NBC, the story of how the Willmar 8 battled the bank

Of all the unexpected places to find it, NBC is presenting one of the most effective labor-management relations films since 1954's classic ''Salt of the Earth,'' directed by Herbert Biberman.

Originally properly titled ''Women of Willmar,'' this new film, skillfully directed by Lee Grant, has been titillatingly retitled A Matter of Sex (NBC, Monday, Jan. 16, 9-11 p.m.).

The story of the Willmar (Minnesota) 8 has been told on PBS in documentary form and it has been hailed in labor and feminist circles throughout the nation. This group of ''average'' women worked in a Willmar bank where they felt they were treated with disdain, paid less than men for similar work, and dealt with unequally in a variety of other ways. In 1977 these women walked out, they picketed, sued, and eventually, although they lost their case in the courts, won their case in the perspective of labor history. Now, according to the show, conditions for women have improved in banks because of the Willmar 8.

As the leader of the group, Jean Stapleton brings warmth, compassion, and a will of iron to the struggle . . . and to the dramatization. Her daughter (Pamela Putch) and the daughter of director Lee Grant (Dinah Manoff) play two of the other strikers with a kind of cool dignity, despite the stresses of their character's personal lives and the pressures of their job action. The script by Joyce Eliason makes very little effort to expand the story except for an unlikely hint of future romance for Dinah.

''A Matter of Sex'' (I wish they would change that title back to ''Women of Willmar,'' since it is the only undignified thing about the whole project) is a minor classic which sets out to tell a simple economic story in human terms. It succeeds gently, with grace, dignity, and a sense of perspective about social victory.