"Mary Hartman, Mary Hartman" is going the way of the waxy yellow buildup on her kitchen floor.

"MHMH," as it has come to be listed in newspapers, will come to an end this summer. Louise Lasser will go on to play in other dollhouses somewhere far from Fernwood, Ohio.

But producer Norman Lear promises us this summer "Fernwood Tonight" a comedy-talk-variety show featuring some of the town's leading lights. In the fall, Lear promises a comedy anthology series which for the moment bears the tentative title of "Fernwood, U.S.A."

The most that one can say about "MHMH" is not that it is ending after 325 episodes, but that it ever got on the air in the first place.

After the idea was turned down by the three major networks. Lear invited a number of individual station managers to come out to his Los Angeles home in August 1975, where he wined them, he dined them and showed them the pilot.

They came, they saw and they optioned. It was a bold move by Lear, going around the networks and setting up what was, in effect, his own network for a series. It is now being carried by 175 stations around the country.

The series immediately became a cult. Louise Lasser became hot copy in the national news and gossip magazines. Summaries of the previous night's episode began to appear in newspapers for those viewers who had missed it. Someone even came out with a Fernwood Flasher doll in time for last year's Christmas gift lists.

At the recent news conference announcing the end of "MHMH," Lear noted that the series had been a phenomenon which by his definition gave it a limited life. He said he wanted to take it off the air before it fizzled out.

"We didn't want that to be 'Mary Hartman, Mary Hartman's' fate. Instead, we chose to leave it as it is, wrap a blue ribbon around it, put it away."

"MHMH" deserves more than a blue ribbon.The people who produce and write television material should give the series a gold key because it opened, in television, the door of taboo that had been locked for many years.

Lear proved in "MHMH," even more than he had in "All In The Family," "Maude" and "Good Times," that you could deal with subjects (adultery, drugs, impotence) once considered off limits and get those shows on the air.

Once Lear opened wide that door of taboo, a great myth was shattered, namely, that you could not put on evening television the mature themes that were being dealt with in movies and books. (I specify evening television, because the daytime serials deal with behavior that not even the Marquis de Sade would include in a lecture to his son about the facts of life.)

If you have any doubts about how much "MHMH," has opened that door, wait until this fall when you watch "soap" on ABC. Its author, Susan Harris, insists that "Soap" is not an imitation of "MHMH." And she is right. It is not.

But the fact that "Soap" is going to appear on a television network this fall is, in my judgment, directly the result of the fact that "MHMH" preceded it. By its success, it opened the door for shows like "Soap" and others like it. My guess is that the networks will never be able to close that door again. Nor will they want to. That is the legacy of "Mary Hartman, Mary Hartman."