SOUTH KOREA took a high-risk gamble in resisting a reasonable compromise over the testimony of former ambassador Kim Dong Jo, who is accused of bribing numerous congressmen, and thereby in forcing the House to make good on its warning to cut off food aid. The aid lost, $56 million, may not mean much to Seoul, a billion-dollar customer of American agriculture. But a barrier has been breached: The United States has found reason - adequate reason, in our view - to take a punitive slap at a longtime ally. It cannot possibly be to Korea's advantage to have its whole future relationship with its principal patron unfurl under the cloud created by its failure to cooperate with the bribery inquiry in the House.

We are persuaded that House investigators went as far as they could to accommodate Korean sovereignty and pride, short of abandoning the requirement laid on them by the House to obtain Ambassador Kim's testimony either under oath or by "comparable means of assuring reliability." But even so, investigators could gain neither the reasonable prospect of agreement on a procedure for taking Mr. Kim's testimony nor, alternately, the Korean government's assurance that testimony however taken would be true.

The best guess is that South Korea figured it would take its licking on food aid in the hope that the House would not go on and take security aid hostage as well. Like everyone else, the Koreans can see that the House is not unrestrained in its enthusiasm for an inquiry that, if it succeeds, promises to vindicate the honor of the House as an institution but to be very costly to certain members as individuals. Indeed, there is a powerful temptation for members of the ethics committee to say they have done everything they reasonably could to promote the inquiry without invading the sensitive area of national security, and to fold the tent.

The Koreans obviously wish to take the food-aid cutoff as the end of the affair, as retaliation - however unjustified in their eyes - for their refusal to meet the terms of the House special prosecutor, Leon Jaworski, on Ambassador Kim. It is just possible, however, that hey have left open a small door. Mr. Kim resigned from his government Friday. Why? It's puzzling. Perhaps he did so to make a typically Oriental apology to his country. But his resignation could affect the question of his testimony: There is a political difference, if not a legal one, between testimony demanded of a government official and testimony offered "voluntarily" by a private citizen.

That prospect alone makes it worth the House's while to continue its quest for reliable testimony. As for Korea, it should not have to be repeatedly or forcefully reminded why it is worth its while for Mr. Kim to respond.