Lucia Chase, the reclusive but formidably authoritative leader of American Ballet Theatre for 35 years, announced yesterday that she will step down as the company's artistic director on Sept. 1, 1980.

Chase, who invested her personal fortune as well as her choreographic and management skills in making ABT an internationally acclaimed company, will remain "in full charge" until then, she said.

It was announced that Chase, 71, and Donald Kendall, chairman of the board of Ballet Theatre Foundation, will jointly make recommendations to the board "to effect a smooth transition to the new (as yet unspecified) artistic management."

Chase's retirement will end an era of artistic hegemony virtually unsurpassed in length and sweep, and from which a legend grew.

Lucia Chase was the model for the role of Adelaide, the sharp-willed, money-obsessed ballet company head played by actrees Martha Scott in "The Turning Point," the 1977 film about the ballet world starring Anne Bancroft and Shirley MacLaine, and featuring stars, guest artists and other dancers of ABT, including Mikhail Baryshnikov.

Agnes de Mille, the distinguished American choreographer who was given her start by Lucia Chase and Ballet Theatre (as the company was originally called) said yesterday however that she found the movie's portrait "very unfair."

"If anyone says a word attacking George Balanchine," she said of the director of the rival New York City Ballet, "it's like insulting George Washington or the Statue of Liberty. But somehow, Lucia has always been fair game for criticism. I think people ought to take another look. She's an extraordinary woman."

The world of dance has known many women as forceful artistic directors -- Ninette de Valois of Britain's Royal Ballet, Marie Rambert, Alicia Alonso, Martha Graham, Celia Franca among them. Even so, Chase stands out for her special combination of dedication, vision and perseverance. De Mille attests to this:

"She can be stern, but she's often had to be," De Mille said. "Her taste isn't always impeccable but it's normal, healthy common-sense taste that people can subscribe to -- all the other companies have gotten attached to one person, one point of view. Lucia's always kept a balance. Sometimes she has dislikes -- she's thrown out ballets of mine, saying she hated them. But she's always been very faithful and courageous when I wasn't in critical favor.

"And she's given America its greatest gift -- a first-class international ballet company, with the most catholic repertoire of any company in the world. The list of her choreographic discoveries and the dancers she's brought to light is simply without camparison."

De Mille means the "gift" part literally. It's no secret any more tht Chase expended large sums of her own to keep the company afloat during its many crises, especially in the early years -- an amount once estimated at $2 million during the troupe's first seven years alone.

Chase was always chary bout revealing any of this. She once said, "When it came out that I gave money... well, that just about killed me!I thought I was disgraced and that my career was over."

She's also habitually underplayed her own role with respect to the company's policies and growth, which most observers attribute to her entrepreneurial abilities. "Let me say... that ABT has never been my company," she declared. "But you ask what I do there. Well, I like to say that I'm the head of the complaint department."

The facts suggest otherwise.Chase was born to a wealthy New England family in Waterbury, Conn., and wanted to be an actress as a child. She studied drama with Rouben Mamoulian at Enw York's Theatre Guild School, as well as music and dancing. When her marriage to Thomas Ewing Jr. was ended by her husband's tragic death in 1933, she devoted herself to ballet in earnest.

She joined Mikhail Mordkin's company as a principal in 1937, dancing leading roles in "Giselle," "La Fille Mal Gardee" and other works. When Mordkin's manager, Richard Pleasant, established Ballet Theatre in 1940, Chase became a chief founding member of the troupe.

Chase continued dancing into the early '50s, creating such roles with ABT as the Eldest Sister in Tudor's "Pillar of Fire," and the Stepmother in De Mille's "Fall River Legend," among others. In recent years, she has continued to appear on stage with the troupe in "Fall River Legend" and and in the purely mimed role of the Queen Mother in "Swan Lake."

In 1945, she became co-director of ABT with designer Oliver Smith, and the two of them tended the company as it introduced and nourished the careers of such dancers asKora Kaye, Alicia Alonso, Hugh Laing, Igor Youskevitch, Erick Bruhn and many others. In recent years, in addition to being home ground for such American dancers as Gelsey Kirkland, Cynthia Gregory, Fernando Bujones and others, the company has also been the main base for such emigres as Mikhail Baryshnikov (before his recnet switch to the New York City Ballet), Natalia Makarova and Rudolf Nureyev.

Under Chase's direction, too, ABT has fostered an immens treasury of choreographic enterprise from Robins, Tudor, De Mille, Fokine, Massine, Balanchine, Feld, Ailey, Tharp and numerous others.Since the opening of the Kennedy Center in 1971, ABT has been recognized as the "official company" of the Centerhs Opera House, where it now currently plays seven weeks annually.