When the Carter administration last year told American banks to suspend their loans to Iran, a major threshold was crossed. Foreign policy and bank loans were publicly mixed. It sent a shudder through the banking community. Would it now be only a matter of time before individual banks were told whether or not they could lend to Eastern Europe, South Africa or Brazil?

Banks are accustomed to the quiet word of advice from their governments or supervisory authorities, but to be told explicitly and publicly what to do is a total different matter.

Two separate events in the last several weeks have highlighted the issue of bank loans and political leverage once again. First came the Polish workers' revolts and with them loud demands, not the least from the leader of West Germany's Cristian Democrats, Franz Josef Strauss, for Western banks to stop giving the Polish government "artificial respiration." Second came the announcement that South Africa was arranging its first public Euroloan in two years. Recently, it has preferred to borrow by stealth, removed from the politicial limelight.

Bankers like to keep their rules simple. After all, they say, when an individual customer comes into a local bank for a loan to buy a new car, he is not asked whether he beats his wife or not. He is merely asked if he can repay the loan in a reasonable period of time. Why should it be any different for a country?

The truth is, however, that collective lending policies of banks have long been influenced by governments. Banks have been encouraged or dissuaded at various times from lending to farmers, home buyers or cooperatives.

The banks themselves have often been quite political. Barclay's Bank, Britain's largest international bank, was drawn into politicial controversy a few years ago when its South African affiliate bought South African defense bonds. After a public row in Britain, the London head office formally criticized the transaction. The bonds were sold.

At the other extreme, a number of U.S. banks, including Wells Fargo and First Pennsylvania, now refuse to lend to South Africa. Partly, they are responding to church, university and other activist shareholders. Partly, they are anticipating political upheavals to come and have decided to reduce their exposure to risk.

The question really boils down not to whether the banks are an instrument of change in the political arena but hown and when they should use their influence.

The Polish situation seems to cry out for positive action. Surely with the economy in such a mess, Western banks should be told to use thier leverage to make sure the economic and political reforms promised to the workers are implemented. A degree of discreet, behind-the-scenes pressure would undoubtedly have a beneficial effect.

On the other hand, for Western governments to go public on this issue would be counterproductive. Dealing with a near totalitarian government gives little room for public maneuver of this kind. If the banks overplay their hand, they will be told to go.

It is a wiser course to try to keep the economic links between Poland and the West intact. One can hope that by enabling Poland to have more contact with the outside world and thus more diversity in its centers of economic decision making, the loans will make more contribution toward liberalizing the economic and politicial climate.

Brazil at first sight looks as if it is completely in hock to the banks. It is the world's largest debtor. It is said to be desperate for more loans to continue the economic growth that will keep its increasingly militant work force in line. If the United States was prepared to consider a cutoff in arms credits to protest against Brazil''s human rights policies, why doesn't it consider a squeeze on bank loans or at least demand that the banks lend to more socially useful projects?

Part of the answer is that Brazil is too big to be pushed around. The Western banks, having lent Brazil $55 billion -- around four times as much as Poland -- need Brazil as much as it needs them. They cannot push Brazil to the point where it might be tempted to renege on its debts.

Moreover, there are lots of heavy borrowers similar to Brazil -- Agentina, the Phillipines, Taiwan, Chile and Mexico. To be consistent, the banks should put the squeeze on them all, but that could ruin the banking system and seriously damage Western export intersts.

Nevertheless, quiet pressure from the banks advising their clients to spend more money on housing and health clinics and less on nuclear power stations and superhighways would serve the long-term interests of both sides. The banks should be able to justify this to themselves by reasoning that this is more likely over the long run to produce a safer haven for their loans.

With South Africa, the argument is rather different. It is a special case. Unlike Brazil or the Phillipines, South Africa has government policies with ramifications way beyond the country. It can be argued that South Africa is a threat to international peace, even a future East-West flashpoint. Therefore, Western governments should take an active interest in what their banks are up to there.

It is worth noting that the church and university boycotts of the last couple of years have been reasonably successful. Apart from two or three, Swiss and West German banks halted nearly all Euromarket lending. It has taken not just the added economic attractiveness of South Africa in the wake of the gold price increase but the liberalizing policies of the government to bring them back in.

Banks are political instruments. They always have been. To use that influence wisely and effectively however is a complicated and danger-fraught process, and the degree of public intervention must depend on each situation. There can be no simple rule of thumb.