FOR SIX HOURS RECENTLY, television viewers had a chance to watch a production described as a "docu-drama." It purported to be the life story of the late Rev. Martin Luther King Jr. and, like some productions that preceded it, it was a curious mixture of reality and fantasy. That, we suppose, is what this new word was coined to describe. "King," as the program was called, was a drama to which documentary interludes were added to create a more powerful impact. Why anyone thought such tinkering with the story of Rev. King was needed is beyond us. Told as a straight documentary or, if you prefer, as a drama, that story is powerful enough. But in this version, film clips of actual events were interspersed with recreations of the same events, the time sequence in which various incidents occured was altered, and some conversations - in particular, those between John and Robert Kennedy - were figments of someone's imagination. Even people who were participants in or close observers of some of the events in Rev. King's life had difficulty separating truth from fiction as the hours rolled on.

To be fair about it, NBC did warn before each night's episode that "in some instances, dialogue, action and composite characters were created to advance the story." But even with such a warning, the program was on dangerous ground. This "docu-drama" merged two of the products television offers to the public - news and entertainment - in a way that made them indistinguishable from each other. By blurring the line, television undermines its greatest public service: letting people see and hear history in the making or in retrospect.

We are familiar with the argument that authors need a certain literary license to make dramas both realistic and interesting. But there is a difference between dramas based on current and past history; where a visual and oral record of history exists, the desirability of fabricating events, conversations and individuals diminishes drastically. There is also a difference between material written for television and that written for the stage or screen. People go to the theater or the movies for entertainment and even children quickly learn to take with a grain of salt the accuracy of historical events presented in such a setting. The same is not yet true of television, largely because of the efforts made by the producers of news programs and real documentaries to stick to the record. But it is likely to become true quickly if the spate of "docu-dramas" and similar productions continues.

"King," of course, is only the latest and most egregious offender. It was preceded by such programs as the one on Jack Ruby and Lee Harvey Oswald, in which at least an effort was made to distinguish between film clips and re-creations, and the one on Sen. Joe McCarthy. Somewhat similar, in a reverse kind of way, was "Washington Behind Closed Doors," in which every effort was made to make a piece of fiction appear to be a piece of history.

Television is much too powerful a medium of communication to be playing so loose with the line between fiction and fact.It is already hard enough to keep them separated. A "docu-drama" is as offensive to journalism and history as the word itself is to the English language.