The Swedish Foreign Ministry is investigating a letter containing new indications that Raoul Wallenberg, a Swedish diplomat who saved thousands of Jewish lives during World War II, may still be alive in the Soviet prison system.

Wallenberg, a center of diplomatic controversy for years, died in prison in 1947, according to Soviet officials.

Since then, however, several persons released from various Soviet prisons have told of seeing Wallenberg or hearing second-hand accounts of his presence.

The letter that suggests he may be alive was purportedly smuggled out of the Soviet Union to a woman who lives here.

Soviet affairs experts in the Foreign Ministry in Stockholm are checking out the letter to determine its authenticity.

The letter was written by Eugenia Kaplan in Moscow to her daughter, Anna Bilder, in Israel. It gives an account of how Jan Kaplan, the husband of Eugenia Kaplan, met "this Swiss or Swede Wallenberg" in the Butyrka Prison infirmary in 1975.

Kaplan had been jailed in 1975 on currency charges. When he was released 18 months later for health reasons, he telephoned Bilder in Israel to tell her of his release. In the conversation, according to his daughter, he mentioned that while in the prison infirmary he had "met a Swede who'd been in different prisons for 30 years and he was in pretty good condition."

The information eventually was given to Swedish officials who asked the Soviets for an explanation. Last January the Soviets told Sweden they had no further information about Wallenberg.

In early July, Bilder received a letter purportedly from her mother containing more information. The letter, dated June 14, had been brought from Moscow by a newly arrived Soviet emigrant.

"I write this letter but I am not sure it will reach you and that the same thing will not happen as with father's letter, because of which he has already spent a year-and-a-half in prison again," the mother's letter said.

"I lost all hope after I was called to the Lubianka prison and learned that all this tragedy occurred because of the letter about this Swiss or Swede Wallengerg whom he met in the prions infirmary...Father wrote a long letter about this Wallenberg and for a long time he carried it around with him looking for a chance to send it to you through a foreign tourist....

"One Saturday, Father came back in a very good mood and told me that at long last he had succeeded in giving the letter to a young foreign tourist who promised to send the letter from Vienna or Germany, I don't remember which."

A few days later, on Feb. 3, 1978, the letter said, Kaplan was arrested again. A Soviet colonel at the prison told Mrs. Kaplan, according to the letter, that her husband had been arrested because, after being freed, he had "ungratefully decided to send out illegally through a foreigner an anti-Soviet spy letter to Israel."

Bilder says she is convinced that the letter is genuine. It is her mother's handwriting, she says, and the mode of expression is her mother's.

Wallenberg's surviving family - a half-brother, Prof. Guy van Dardel, and a half-sister, Nina Lagergren - have been forming committees in various countries in recent months to press the Soviets to make a fuller disclosure of Wallenberg's fate.

Wallenberg, who if alive would be 65, was sent to the Swedish legation in Budapest late in World War II to save Jews. He issued thousands of special Swedish passports to Hungarian Jews, enabling them to escape Hitler's death camps.

He was last seen outside the Soviet Union when, shortly after Russian troops moved into Budapest, he was taken to a meeting with Marshal Rodion Malinovsky, commander of the Soviet forces.