You had probably best know about this. "Brideshead Revisited" begins an 11 1/2-hour series tonight at 8 on Channel 26. It is too good, I think, for public television.

But assuming they will not dare start their periodic groveling for contributions from their audience in the best series they have ever had, you may very well find it entertaining. You may even find it a little beautiful.

It is, without splitting hairs, the best series ever seen on American television and it is, needless to say, not American; for we in America distrust always the giving of the best we have got to our fellow countryman. We forever hold back, on the theory that our neighbors are not so wise or remarkable as we are.

No American show is likely to commence the introduction of two main characters by having one of them vomit through the window of the other. To begin with, we would say "upchuck." And to end with, we would change the scene itself, to something a little more acceptable to what we presume is the taste of the masses.

In America, very few artists, and almost no operators in the big bucks, have ever trusted the guy in the undershirt out there. In America, we think other Americans are stupider than we are. This leads to a falsity of tone and to a holding back of love.

These vices, so peculiarly our own, are voided in "Brideshead." They give it all they've got, and the result is fairly overwhelming. It will be a very great success in our country, for we are not used to seeing much on our idiot boxes.

It is the story, to begin with, of a romantic friendship between two young men. If you want to say faggot, or fling a bit of opprobrium, I guess you could.

I do not think you will say it, nor do I think you will laugh at them or despise them, if you watch it.

Something a bit remarkable occurs in this show, and I suspect the first rumbles of praise will come, not from the intelligentsia, who in America are so commonly stupid, but from the grass roots.

The two guys can't get over each other. It is not a porn thing -- reviewers rarely see exceptional wit or merit in that -- but it is a discovery thing.

Their friendship falls apart, not as so many youthful and tentative friendships do, but at a level somewhat serious and somewhat terrible.

The hero of the piece, the narrator, played by Jeremy Irons, is an obviously straight arrow -- not that that helps -- who first comes alive with his friend, Anthony Andrews.

There is something rather Irish about it, or at least rather youthful and naive. There is innocence and there is love, and I don't think anybody will be unduly corrupted in tonight's beautiful two hours. (The series runs in one-hour segments, except for tonight and for the final one of an hour and a half, which we shall see 11 weeks from now.)

Time does not stop his clock for us, and he does not stop it for them. Life goes on. It gets much worse, in succeeding hours. There are marriages, affairs, false starts and blockages of careers. There are deceptions, there are foolish decisions. Unless you are quietly dead, these will not distress you or shock you. Or not more than you know is right.

Physical beauty falls quickly, as it does in the world. Lyric turns sordid. Things go from bad to worse.

It is a show, however, about grace.

Okay. Men turn pigs. Women turn faithless. Guardians turn rats. Everything is lost.

Except, in the ghastly mixture, something starts burning, like one of those stars on the world's first morning. It is radiant enough to amuse us and sparkling enough to win our praise at the last.

It is a beautiful show. First, because of the author, the late Evelyn Waugh, who was a far better writer than we Americans are used to, and that means far more near the center of common life.

Too much will be made, if you listen to loons, of the religious aspect of the story. The author would have been shocked to hear imbecile critics say it is a Roman Catholic story, when it might just as easily have been about Baptists or Holy Rollers or anybody else. Presbyterians? Sure, why not? It is about grace, and about that familiar ginger of the human heart that turns fate, turns disgust, turns awful pain, into something else. Which is sometimes merry, sometimes bearable, and invariably recognizable in ourselves.

I suppose if you had to sum it up in a nutshell, you could do it in a sentence: "Though I make my bed in hell, even there Thou art with me."

But I see no reason to be grand or profound or glum about it. It is funny as hell. I will not ruin your pleasure by speaking of the endless scenes, the infinitude of small characters, that will very certainly set you laughing.

The acting -- Laurence Oliver and Claire Bloom are among the enchanters -- is of a quality so high you will probably not believe it. Endless details are tended to -- by the usual means of endless work, endless pushing for a standard far beyond anything required to date on television -- and these include the music, the settings, the timing of responses.

You should be warned you are going to have to go down, from the early lyricism and innocence, to a level that hurts you, but which I think you will know a good bit about, yourself.

The series, I think, is not going to cheat you or play it double with you, and you will like it, if you are of my mind, because it is trying to give you something, rather than squeezing out from you what you can give it, in the way of easy guffaws. This is not the series you will peer at dimly, wondering if you should call old Aunt Mildred, or just keep watching, yawning.

Its laughter, therefore, because it is level with you, is a more wholesome laughter than usual, its portrayal of pain less simpering than usual, and its ultimate cheer is --for a change -- fairly believable.

Claire Bloom is admirable as a woman who loves the swan-neck of virtue but who knows nothing of its sweaty hair. She is despicable, charming, pitiable by turns. Olivier is startlingly fine as an old goat who retains less duplicity and more strength than usual. John Grillo plays a disgusting academic who lies and cheats and flourishes, at least for a time; and Diana Quick, in the more conventional role of a woman who marries wrong, who tries to bring order to her life, and who does not quite have the love that would make things turn out right for anybody, is impressive as one who is rather full of baloney and somewhat lacking in the ultimate thrust into the unknown, that alone would save her. Nickolas Grace plays a homosexual queen to the very limits in a tour de force display of acting; and the two leading characters of tonight's opening are astounding in their sureness of tone and general flawlessness.

The pleasant fact is that Granada Television (London) not only had a good idea of what you would happily accept, but also the technical skill, the financial faith, and (somewhere amid the corporate mistiness that afflicts all great institutions of the mass media) the cleanness of heart to give a show worth saying thank-you for. Let me be one to say it. Thanks. And I find it a little unsettling that commercial television in a collapsed (?) empire has outdone in vigor, style and substance the work of television in our rising (?) republic.