It is 9:30 a.m., and a guard in a brown uniform slides back a pair of heavy metal doors. The main branch of Nanyang Commercial Bank, located on the edge of Hong Kong's Central District, is open for another day of business.

Within minutes, the first customers are standing before the tellers' windows. From appearances, they seem to be a cross section of local society: prosperous businessmen, a housewife, company messengers -- all anxious to finish and be on their way.

These customers have selected a bank that is ordinary in every way but one. It is a Communist bank, one of a large network that Peking operates at a profit in one of the world's most intensely Capitalist economies.

Through Nanyang and 12 other banks either controlled or owned outright, The Peoples Republic of China will skim millions of dollars this year off Hong Kong's fast-growing financial sector.

The banks are coordinated from an imposing 17-story building standing on some of the colony's most valuable real estate. It is local headquarters of the Bank of China, Peking's major repository for foreign exchange.

Inside, beneath portraits of Chairmen Mao and Hua, clerks and tellers eagerly engage in that very un-Communist pursuit, the creation of profit by moving money.

This pragmatism is by no means a result of the liberal prrsuasion of China's current leadership, although the new policies do seem to have given the banks more flexibility in selecting cilents.

The fact is that China has made money this way since the day the People's Republic was founded in 1949 and straight through the turbulence of the Cultural Revolution.

In Hong Kong, one is never far from a Communist or Chinese national bank, as they prefer to be called. They operate about 130 branches, or one in 7 of all branches in the colony.

One can pick them out by the golssy photos of steel mills, Chinese acrobats and party dignitaries customarily displayed in their windows.

The banks' officials offer no precise figure for the size of their aggregated balance sheet. But analysis of available data, plus considerable guesswork, suggests they hold something over 10 percent of the local banking system's $27 billion in assets.

They have attained this stature by predicating most business on politics. Their major clients are the large collection of department stores, insurance companies, oil retailers and trading firms that china operates in Hong Kong.

Together the banks and companies are estimated to earn about $2 billion annually. But in the banks' case, profits are only one aspect of their usefulness. Equally important is that their deposit base is a freely exchangeable foreign currency.

Hong Kong dollars placed on deposit with the banks are theirs to use until withdrawn and allow wholesale purchase of U.S. dollars, yen or whatever currency China's foreign trade requires.

Politics can figure in dealing with ordinary Hong Kong people and companies as well. Some individuals like to place their money with "patriotic" banks. Some companies might feel accounts with them will help when Peking's local companies award contracts.

However, for many customers -- just how many is hard to say -- the banks are banks like any others. Depositors might find their branch locations convenient or feel their service is a little more personal.

They might want to open a time deposit in renminbi, the mainland's "people's currency." These accounts' interest is tax-free because the renminbi that the depositor buys with Hong Kong dollars are in theory held in Peking, where there is no tax.

These people are offered every service found in an ordinary bank: checking accounts, savings accounts (in U.S. dollars, if desired), consumer loans, demand drafts. One bank, Po Sang, even specializes in buying and selling gold. gold.

In recent months, Peking appears to have instructed its managers to think less about politics and more about money, and to court these customers more aggressively. At the same time, the banks have entered the real estate market, formerly considered too risky and ideologically distasteful.

"In the past six months, the P.R.C.-controlled companies have invested quite heavily in local real estate," noted a university professor who long has studied Hong Kong's financial scene. "And they rely on financing from the P.R.C. banks."

Their largest such venture to date is a residential and commercial estate being built over a subway station. One of the firms developing the complex is The Sun Co., known to have strong links with China. Its share of the financing, reported to be about $85 million, was put up by Nanyang Commerical Bank and Bank of China.

Participation in a long-term, bricks-and-mortar project of this size was taken by local political observers as another sign that China wants Hong Kong's status quo to remain in place indefinitely.

The bank also have become less wary of Hong Kong's often volatile stock exchanges. According to the professor, the banks now accept shares as collateral for loans, but are standing by their refusal to finance purchases of shares.

The banks' directors, who normally share the penchant for secrecy found in all Chinese government agencies, are being more sociable with colleagues in other banks (but remain inaccessible to most journalists).

"Before, those fellows would not talk," remarked an economist at an American bank. "How they call me and ask, 'How's the U.S. dollar today?

Ostensibly owned by individuals in China, four of the 13 banks are incorporated in Hong Kong and as such are required to disclose accounts and ownership information once a year.

Nanyang is one of these. Its shareholders include such names as Seng Min Ho, merchant, of 8 East Iron Brigade End, Canton, who is shown to personally hold most $2 million in shares.

Analysis of their accounts reveals generally conservative, but profitable management.

For example, in a recent study of Hong Kong's 34 locally incorporated banks, Nanyand ranked as the fourth most liquid (62 percent of deposits in liquid form) and had the second lowest ratio of loans to deposits (42 percent).

Another Communist bank, Hua Chiao Commercial, was the fifth most liquid (54 percent of deposits) and the fifth most profitable in terms of net earnings as a percentage of average net worth (25 percent).

All indications are that the banks anticipate expanded business. To assure that they don't get buried in paperwork, they are buying U.S. computers. Under a contract signed with Bank of China, International Business Machines Corp. is supplying on-line facilities that will give tellers in all 13 banks direct access to central accounts computers.

IBM itself has released very few details about the job. But computer specialists say it has had trouble getting the purchaser to disclose accounts information that is normally needed to design a system of this size.

Business is looking up partly because China, like its banks, is buying increasing volumes of foreign technology, much of it through Hong Kong. Because imports require foreign currency, the banks' prosperity would seem secure so long as the buy-foreign trend continues.