Newsweek, Inc. will begin test publication next month of a new bi-monthly magazine called Newsweek Focus specializing on one subject each issue, Newsweek Chairman and President Peter A. Derow said yesterday.

"Newsweek covers the breadth of the world's news each week," Derow said. "Newsweek Focus will provide a new dimension of depth. In so doing, it will take the Newsweek editorial franchise beyond the confines of its present weekly frequency and editorial format."

Newsweek Executive Vice President Mark M. Edmiston said that the new publication grew out of requests of some Newsweek readers for more information on topics.

"Even when the magazine devotes as many as 20 pages to a cover story, hundreds of readers write our editors asking for even more information," Edmiston said. "Newsweek Focus has been developed to give them that information."

The magazine will be tested in Atlanta, Boston, Chicago, Dallas-Ft. Worth, Kansas City-Topeka, Minneapolis, Philadelphia and San Franciso next month where 140,000 copies will be sold at news stands. The price will be $2.50 in Atlanta and $2 elsewhere, the company said.

About 10,000 more copies will be distributed in Amsterdam, Caracas, Hong Kong, London, Mexico City, Paris, Singapore, Sydney and Tokyo, the company said.

The first issue, consisting of 108 pages, two-thirds of which will be editorial material, will be devoted to the mysteries of the cosmos and will include recent discoveries about the universe and its future.

The company said it will decide later this year whether to publish the magazine regularly.