Berthold Altmann, 75, a researcher and author who specialized in medieval history, died Saturday at his home in Alexandria after a heart attack.

As a historian, he worked at the Harry Diamond Laboratories in Washington from 1956 until retiring in 1972 as chief of scientific and technical information.

Mr. Altmann's interest in medieval history dated back to his earlier days in his native Germany. He received a doctorated in history and languages at the age of 21 from the University of Freiburg. He also held a master's degree in library science.

He was a historian for the National Library of Germany and concentrated on studies involving the origin of the medieval city in Germany.

Mr. Altmann came to the Washington area from Germany in 1939. A short while later, he entered the University of Chicago where he earned a second master's degree in library science.

He then returned to Washington, where he worked as an editor in the Department of the Army until joining the Diamond Laboratories. He was a retired lieutenant colonel in the Army Reserves.

Mr. Altmann published more than 50 books on medieval history and wrote numerous articles for historical scientific and Masonic publicatians.

He was a member of the Masons, the American Historical Association and the American Association for Medieval History.

He is survived by his wife, Margot Altmann, of the home; a son, Dr. Ruben Altmann, of Council Bluffs, Iowa, and five grandchildren.