Shoji Sawada, a man of grace and hardihood, remembers the last words with his mother. It was on the morning of Aug. 6, 1945, in Hiroshima, Japan. Mother and son, ages 36 and 13, were at home less than two miles from the blast center of the atomic bomb that the American military exploded 40 years ago. Sawada's mother was one of about 130,000 Hiroshimans -- in a city of 350,000 -- who died in the indiscriminate bombing.

In the wreckage of broken wood, plaster, windows and a crushed roof, Sawada recalls that although he didn't know what was happening, it was happening instantaneously. One moment he was asleep in bed, the next awake in devastation. He was able to crawl out of the pile of rubble that only minutes before was his bedroom. In a clearing, he looked at the horror. He breathed in ''the yellowish air.'' Small fires were breaking out.

''I heard my mother call my name,'' sawada says. ''Her voice seemed to be coming from far away, though I knew there was not much distance between her and me . . . My mother said her legs were caught in big beams or pillars, being unable to move. I tried to pull out the broken pillars and struggled tp push up the plasters with all the power I had. But it was far beyond my ability. I called for help to adults in vain, because those wounded people could do nothing more than find a safe place for themselves.''

Fires were spreading. As they came closer, Sawada's mother called out, ''That's enough, never mind your mother. Get away from here, right now.''

The boy, terrified, obeyed. Outside of what was once his home, he saw burned and blackened bodies. He moved farther away. The smoke from the fires became a cloud: ''When I thought about my mother under the smoke, I felt my heart was broken. 'Couldn't I really help her? There must be a way.' I had tears of regret and repentance in my eyes. Even now, 40 years after, the same feeling hits me, and I am depressed whenever I think of my mother.'' She burned to death.

Four decades of mental torture have passed for Sawada. Today he is a physicist and faculty member at Nagoya University. He is one of 370,000 Japanese survivors who -- as Aug. 6 approaches, as well as Aug. 9, the 40th anniversary of the bombing of Nagasaki -- have wondered about the accuracy of the words inscribed in the memorial cenotaph in Hiroshima: ''Rest in peace, for the mistake shall not be repeated.''

As part of his commitment against that repetition, Sawada visited the United States last month. He came with a delegation of other Hibakusha -- ''explosion-affected persons'' -- to rally Americans to work harder to abolish nuclear weapons.

The group laid a wreath at the Tomb of the Unknown Soldier and met with several U.S. peace groups. In a letter written two months before their Washington visit, the Hibakusha appealed to President Reagan for a meeting. Reagan refused, as did Vice President Bush and Secretary of State George Shultz.

Other Japanese visitors had better luck in swinging open the gates of power. On the day that Sawada and his fellow survivors were praying at Arlington Cemetery, George Shultz was posing for promotional pictures at the State Department with a group of sumo wrestlers.

In the two evenings I spent with Prof. Sawada and his colleagues, I was struck by their extraordinary personableness. They were militantly pro-peace and anti-war, yet they expressed no resentment against those who created the hell they had survived. They were familiar with history and surely knew that when Harry Truman was handed a 26-word message that the bomb had just been dropped on Hiroshima, he proclaimed proudly that ''This is the greatest day in history.''

Sawada is also free of declamations about those political leaders who threaten to Hiroshimize the planet, not merely a city. He and other Hibakusha offer themselves as witnesses to the severest form of destructiveness man has yet to devise. A delegation has gone to the Soviet Union as well as the other nuclear nations. All have been delivered the same message: ''We are well aware that the atomic bombs were dropped by the United States in the war which was provoked by the militarism in Japan, and with which we ourselves did collaborate . . . There is no way out of this nightmare unless people all over the world realize through the testimonies of us victims that nuclear weapons cannot exist side by side with humanity, that they are by nature inhumane.''

The words seem understated compared with the global annihilation that is a daily threat. The Hibakusha, carrying news of life-destroying demons at the ready in planes, submarines and silos, have spent 40 years trying to get the world's attention. They have yet to tire. At times, as Sawada confessed, they are amazed at how few people want to listen.