Two forces that shape the news Americans see, hear, and read are headed toward a collision. So says media critic Ben H. Bagdikian, a former Pulitzer Prize-winning reporter and one-time ombudsman and assistant managing editor of the Washington Post.
He describes the first trend - the continued purchasing of newspapers, television and radio stations, magazines, and book publishers by large corporations - as an alarming concentration of the media in the hands of a few. But resistance, he says, may come from a second trend, this one toward greater professionalism and ethics among writers and editors who work for these giant corporations.
In a book released Monday titled ''The Media Monopoly,'' Mr. Bagdikian asserts that 50 large corporations now control the majority of America's print and broadcast media. These owners, he says, restrict what news is presented to the public either overtly or covertly in order to guard their own interests. Among his findings:
* Twenty newspaper chains now control more than half the daily newspaper sales. In 1900 there were 2,042 daily papers with 2,023 owners. By 1980, there were 1,730 dailies and only 760 owners.
* Twenty corporations have just over 50 percent of the annual magazine sales.
* Ten corporations have over half the audience for commercial radio.
* Eleven firms received over half the $7 billion in book sales for 1980.
* Board members of major newspapers and TV networks also serve on the boards of some of the largest US corporations.
* Ninety-eight percent of all newspapers are the only papers in their communities.
Bagdikian says these ''interlocks'' between corporations and the media have caused changes in the way events are reported. ''Some subjects, some ideas,'' he said in an interview, ''will have more trouble getting in not because of the inherent nature of them, but because it will be offensive to the owners. . . . In the end there is a different flow into society of ideas and certain subject matter. And that distorts the total picture.''
A survey by the American Society of Newspaper Editors found that 33 percent of all editors working for newspaper chains said they wouldn't feel free to run a story that was damaging to their parent firms. ''I think any reporter who works for a commercial publication has had . . . the experience of having a story killed because it offends the publisher or the interests of the owner,'' Bagdikian says.