With a single en bloc acquisition of 850 early editions - 516 printed before 1531 - the Folger Library has become a premier center for Reformation studies, adding a new dimension to Washington's cultural resources.

The purchase, which quite properly can be called an exciting coup in the scholarly world, was announced yesterday by O.B. Hardison Jr., Folger director. He called it the Library's most important acquisition in a decade.

What Folger has acquired has been described by experts as the most extensive and interesting private collection of Reformation works assembled since the turn of the century.

It had been brought together by a Swiss bibliophile and was offered exclusively to Folger by the firm of Erasmushaus-Haus der Bucher. The firm, which dates back to the 16th century and was in existence when some of the collection's books were being printed, has its offices in the house in which Erasmus once wrote his treatises.

Many of the volumes, illustrated by early 16th-century German masters of the woodblock, also offer artistic riches. Attributions include Hans Holbein, Lucas Cranach and Hans Boldung Grien.

In the Martin Luther titles alone, the collection adds 180 new items to Folger's collection of 300 titles and makes the Library's collection perhaps the finest in the United States. One of the prize items is a 1518 expanded version of Luther's 95 theses, printed a year after he nailed them to the wooden door of the church of Wittenberg.

Friends of the Folger Library will have a private viewing of the new acqusitions at their annual meeting tonight at Folger. Before they go on public exhibition, they will have to be catalogued and accessioned into the library's collection.

The purchase price was not made public but came from Folger's capital $99;[WORD ILLEGIBLE] large anonymous gift by one of [WORD ILLEGIBLE] friends and grants from the Crystal Trust of Wilmington, Del., and the National Endowment for the Humanities.