Romy Schneider, 43, an actress who appeared in more than 50 films in a career spanning nearly 30 years, was found dead in the apartment of a friend here May 29.

An official at the office of the public prosecutor said Miss Schneider "apparently suffered some kind of cardiac arrest." No specific cause of death has yet been given. Miss Schneider had undergone a kidney operation last year, but was not known to be suffering from any illnesses.

She was born in Austria, the daughter of Austrian actor Wolf Albach-Retty, and a German actress, Magda Schneider, a star of some 60 German films.

Miss Schneider's film career began in Berlin at the age of 14, when she played the daughter to her real mother in "White Lilies." Most of her films were French or Italian.

Miss Schneider acquired international fame as "Sissi," the Austrian princess heroine of a series of light-hearted romantic films.

In 1958 she met French heart-throb Alain Delon and fans avidly followed their five-year romance.

She made her Hollywood debut in "Good Neighbor Sam," with Jack Lemmon and Dorothy Provine. Other films included, "What's New Pussycat?", "The Trial" of Orson Welles and Otto Preminger's "The Cardinal." She also played a violin-playing trollop in a war film called "The Victors." She worked in 1962 with Italian director Lucino Visconti in "Boccaccio '70."

Since the late 1960s, she appeared in a string of films with Costa-Gavras, Claude Sautet, Claude Chabrol, Bertrand Tavernier and all major French directors, bringing Miss Schneider her first Caesar (the French Oscar) in 1976 for a role as a luckless actress, and another three years later.

She had recently completed "La Passante du Sans-Souci," in which she played a mother whose son dies. The movie carried a dedication to her late son, David Christopher Haubenstock, when it was released. Her 14-year-old son died last July when he fell on a wrought-iron fence and impaled himself on a post. He was the child of her first marriage to West German director and actor Harry Haubenstock, who used the stage name Harry Meyen. Haubenstock committed suicide in 1979.

The accident in which her son died occurred at the family home of Miss Schneider's second husband, Daniel Biasini, a photographer who had been her personal secretary. They married in 1975, separated in 1977 and were divorced last year.

Miss Schneider had lived in France since the 1960s. Her survivors include a 4-year-old daughter, Sarah, by her second marriage.