A government prosecutor dropped a murder charge yesterday against the Silver Spring man accused in the fatal stabbing of 21-year-old Cynthia Louise Herbig, the former Radcliffe College student turned prostitute who was killed last year near Dupont Circle.

The man charged with her slaying, Albert A. (Kit) Breach, 32, was scheduled to go on trial yesterday morning in D.C. Superior Court. However, Assistant U.S. Attorney Donald L. Golden walked into court and without giving any explanation asked Judge Carlisle E. Pratt to dismiss the second-degree murder charge against Breach.

The prosecutor said the government intends to pursue other prostitution-related charges against Breach, who allegedly persuaded Herbig and five other women to work as prostitutes for him. Golden and other prosecutors also declined after the brief court hearing to explain why the murder charge was dropped.

How Cindy Herbig -- an accomplished cellist and honor student from Missoula, Mont., who won a scholarship to Radcliffe College -- ended up working as a prostitute on the streets of Washington before she was killed is a question that has baffled her family, friends and prosecutors.

How and why she died is a question prosecutors had hoped to determine during the Breach trial. But for now, anyway, the slaying remains unsolved.

Herbig, who used the street name "Mika Jenson," died of stab wounds in the chest and abdomen at George Washington University Hospital on Jan. 17, 1979. She had been taken to the hospital by Breach, who told police he found her lying on the sidewalk outside her apartment building near Dupont Circle.

A D.C. Superior Court grand jury indicted Breach on the second-degree murder charge in August 1979. He was arrested two months later by Canadian police in Calgary, Alberta, Canada, where he apparently had gone sometime after the killing. Breach has been held in the D.C. jail for the past 11 months awaiting trial.

Herbig, who grew up in Missoula, played the cello and graduated third in her high school class. She kept up her interest in classical music at Radcliffe, where she pursued studies in science. According to friends, however, she became disenchanted with the academic demands and Ivy League atmosphere of Cambridge and returned to Montana in 1976 during her sophomore year.

Breach met Herbig in Missoula and she accompanied him back to Washington in mid-1977 and started work as a prostitute.

She apparently was very successful. Two of her "trick books" were confiscated by police and the books listed hundreds of names of her customers. One source who has seen the book said it included prominent attorneys, local officials, professors, and others, many of whom were from out of town.

While working as a prostitute, often in the 14th Street corridor, she continued to pursue her love for music. One of her favorite pieces, she told an acquaintance, was Pablo Casals' 1933 version of Dvorak's Cello Concerto.

Court papers filed by prosecutors indicate Breach allegedly had threatened Herbig when she said she wanted to leave him. As recently as a week before the slaying, she confided in one person that she was intending to return to school, and leave Breach, with whom she lived at times in her apartment building in the 1300 block of 22nd Street.

She told one man she met at a party that she liked the money she earned as a prostitute and was comfortable with the life style.

When Herbig's body was found near where she apparently had been attacked, she was wearing a rabbit's fur jacket, a skirt, and one boot. Her purse, its contents, and her other boot were lying in a parking lot nearby. According to a source, among the contents of her pocketbook were numerous slips of paper containing the names of her most recent customers, and the amount of money they had paid her.

The news of her violent death came as a shock to the residents of Missoula.

An overflow crowd of family and friends attended her funeral and paid respects to her parents. Her father is the director of the Missoula Youth Symphony. The local newspaper came under intense criticism from readers for revealing details of Herbig's life as a prostitute.

Students who had known Herbig at Radcliffe and Harvard University were stunned by the news. The Harvard Crimson, the student newspaper, said she had chosen an "alternative life style" and carried a school photograph of the blond Herbig, dressed in painter's overalls standing with a group of her classmates.

According to court papers, some witnesses said they saw a car leaving the parking lot where Herbit had been stabbed. The defense theory was that the car's driver, who it contends was not Breach, might have been the actual suspect. It was not immediately clear yesterday whether prosecutors had dropped the murder charge because they had discovered new evidence bearing on the case or another suspect.

Herbig, who was described by women in the 14th St area who knew her as "sweet" and "conservative" in her appearance, was apparently unusually careful about her business. The looseleaf book in which she kept a record of regular customers, however, apparently provided no decipherable clue as to who else might be the possible suspect who Breach's attorney says may be involved.

According to one source, perhaps the most telling items that would have been entered into evidence had the trial been held were a series of photographs of Herbig that seemed best to describe the last few years of her life.

The photos range from her Missoula driver's license, picturing a small-town teen-ager with pigtails, to her Radcliffe student identification card.