BERLIN, March 13 -- Americans with PhDs beware: Telling people in Germany that you're a doctor could land you in jail.

At least seven U.S. citizens working as researchers in Germany have faced criminal probes in recent months for using the title "Dr." on their business cards, Web sites and r¿sum¿s. They all hold doctoral degrees from elite universities back home.

Under a little-known Nazi-era law, only people who earn PhDs or medical degrees in Germany are allowed to use "Dr." as a courtesy title.

The law was modified in 2001 to extend the privilege to degree-holders from any country in the European Union. But docs from the United States and anywhere else outside Europe are still forbidden to use the honorific. Violators can face a year behind bars.

Ian Thomas Baldwin, a Cornell-educated researcher at the Max Planck Institute for Chemical Ecology in Jena, has stopped calling himself "Dr." ever since he was summoned for interrogation by police two months ago on suspicion of "title abuse."

"Coming from the States, I had assumed that when you get a letter from the criminal police, you've either murdered someone or embezzled something or done something serious," said Baldwin, a molecular ecologist. "It is absurd. It's totally absurd."

No one has questioned the legitimacy of his degree or whether he has the right to conduct research here. But going by "Dr." is verboten. If he wants to refer to his doctorate, German law dictates that he identify himself as "Prof. Ian Thomas Baldwin, PhD, Cornell University."

Baldwin confessed in a telephone interview that "there's no question I'm guilty as charged." But he hopes prosecutors will give him a break.

In his defense, he noted that the Max Planck Institute has always addressed him as "Prof. Dr. Baldwin" since it offered him a job a decade ago, and nobody warned him he might be in legal peril if he did likewise.

The proper use of honorifics is no small matter in Germany, a society given to formality where even longtime neighbors insist on addressing each other using their surnames. Those with advanced degrees like to show them off, and it is not uncommon to earn more than one. A male faculty member with two PhDs can fully expect to be called "Herr Professor Dr. Dr. Schmidt," for example.

In effect, forcing Americans to forsake their titles amounts to a social demotion. "It's an indication of the hierarchization of German society," said Gary Smith, director of the American Academy in Berlin. "Germans are much more status-conscious about these things, and the status is real."

Smith holds a doctorate from Boston University and has tempted fate by answering to "Dr. Smith" during the two decades he's lived in Germany. He said he was told years ago that there is a legal way for foreign PhDs and MDs to register for permission to use the appellation, but he has never bothered.