IT'S BEEN instructive to have President F. W. de Klerk of South Africa in town. He has displayed an earnestness and identification with change reminiscent of the mid-passage Gorbachev. Mr. de Klerk and Nelson Mandela, his former prisoner, are now the principal negotiating partners in an unprecedented effort at national rescue. The goal is turning that country, socially and morally deformed by apartheid, from an unjust society into a just one.

President Bush, receiving the first South African president to visit the White House since apartheid became ruling doctrine, was impressed by Mr. de Klerk. He pronounced the magic word -- "irreversible" -- to characterize the change underway. The word has become code for the degree of progress toward nonracialism that would meet the broad international condition for ending punitive measures against South Africa and resuming normal ties. Mr. de Klerk has begun the relaxing and dismantling of apartheid's restrictions on blacks, and he has opened a multiracial political process. These changes and the energies they have released justify Mr. Bush's hopeful judgment.

Some observers of Mr. de Klerk remain skeptical. They observe, accurately, that his government in its legal and institutional essentials is still an apartheid regime. They point to disquieting signs that have appeared in the terrible recent violence that Mr. de Klerk may not yet have all the security forces aboard the reform train. They fear giving him any incentive to pocket his current gains and slow down.

There is a lever available to strengthen Mr. de Klerk's reform hand, if it needs this sort of exterior strengthening. American sanctions legislation requires delivery on a range of forward steps, and, as Mr. de Klerk himself understands, he isn't there yet. For now, sanctions should stay on. Mr. Bush made clear, however, that if and when Mr. de Klerk delivers -- it may be a matter of months -- the sanctions will be lifted as American law prescribes. The best way for Americans to strengthen reform, after sanctions, is by a vigorous economic role.