It was August 13, 1950. W. Averell Harriman was returning home from one more delicate mission--as President Truman's emissary to General Douglas MacArthur. He was about to take up another dicey piece of business as Truman's foreign policy coordinator.

Reporting in a late evening "newsmakers" broadcast, Walter Cronkite had this to say of the former almost- everything in American politics and diplomacy whose 90th birthday tomorrow was dedicated this week, at a fund-raising dinner, to the fortunes (material as well as political) of his beloved Democratic Party:

"Harriman could have been a playboy. He was born with a $70,000,000 silver spoon in his mouth . . . went to Groton and Yale . . . (then) into banking, became an eight-goal polo player, even developed a sort of upper-class stoop, a slight bow to boredom. But this millionaire was made of considerably sterner stuff . . . . Today Harriman seems to be a man devoted solely to selfless service to his country."

"Selfless service"--it sounds smarmy, unless you happen to be fed up to here with the rancid condition of American statescraft as it comes across these days: the obsession with "turf"; the "guerrilla warfare"; the who's-up-who's-down- who's-on-the-skids speculations of breathless analysts. Against that backdrop, a review of the record of Averell Harriman shines bright as a timely reminder to today's pushers and shovers of the purposes they are supposed to be serving--of what public service, at its best, is all about.

What it was all about, in Averell Harriman's case, was almost a half-century of reflexive, fire-horse pawing and champing at the fire bell's first ring. When Cronkite spoke of a "tall, stooped, unsmiling man who wearily climbed down from the big Constellation at Washington's National Airport," Harriman was 59 at that time and "wearily" was apparently the operative word. The way people talked about him 31 years ago made Harriman sound sometimes like a prime candidate for early retirement.

New York Times columnist C. L. Sulzberger, in an album of memoirs entitled "A Long Row of Candles," offered these snapshots of that period: "When Averell came in, he looked ghastly. He is visibly overtired and thin . . . (and on another occasion) He looks tired and worn . . . (and on another) He really looked bushed . . . and again) He looked poorly and has a tic in his eye . . . (and in 1951) Last week he worked straight through from 8 a.m. until 7 a.m. the following morning. That's too much for a man of sixty."

As it turned out, it wasn't. He had already served two presidents over 17 years: Franklin D. Roosevelt, as New Deal manager, lend-lease coordinator in London, wartime ambassador in Moscow; and Harry Truman as promoter and roving ambassador for the Marshall Plan. Still ahead was a mind-boggling succession of jobs: ambassador to London, secretary of commerce, nuclear test ban negotiator, undersecretary of state, Vietnam peace negotiator, and a half- dozen special presidential missions-- not to mention governor of New York.

He voted for Al Smith in 1928 and became a Democrat for life because he was "very disturbed by the fallacies of the Republican Party, both domestic and foreign," he explained in a long conversation the other day. "I am just about always disturbed about something--you have to be."

He says he "can't get as excited about some things as I used to when I could put on a hat and do something about it." But when I called on him, he was hunched up, inches away from his television set. Israeli Prime Minister Menachem Begin was holding forth on ABC's "Issues and Answers," and if Harriman wasn't excited, his muttering suggested that he was, let's say, disturbed.

He was also philosophical. "Every situation always looks the worst it's ever been," he replied, when asked about the current international scene. "But these questions get solved because they have to be solved."

Does the man who dealt with Roosevelt and Churchill and Stalin subscribe to the theory that these men were giants--and that there are no giants on the contemporary stage? "History makes them giants, because what they do becomes important," is his answer. "But they are not giants when they do it--there will always be somebody around to do what's needed."

Without vanity but by way of illustration, he recalls that "everybody thought I was crazy" to accept a come- down from undersecretary of state to assistant secretary in the 1960s. "But it didn't bother me, because I knew I had a job to do. When something needed doing, I usually started to do something about it." He doesn't think that's unique--"there are many Americans who can do something about these things."

He's probably right. But there are not many--or at least not all that many now in action--who fit the test of selfless service set down by George Kennan, Harriman's No. 2 man in Moscow in the war years. In his "Memoirs" in 1967, Kennan wrote of Harriman:

"No diplomatist ever executed more punctiliously (his) instructions . . . . None--be it said to his eternal credit-- was ever less inclined to distort the record however imperceptibly, in order to show himself and his performance off to good advantage . . . . The United States has never had a more faithful public servant."