Six major Texas banks, suffering from falling oil prices and declining real estate values, yesterday were placed on a special "watch list" by Standard & Poor's Corp., the major New York bond-rating company.

Standard & Poor's said it is examining each of the six bank-holding companies and "generally expects minor ratings downgrades to result from this review."

Four of the bank companies cited by the rating agency -- RepublicBank Corp., MCorp, Interfirst Corp. and Texas Commerce Bancshares Inc. -- have assets in excess of $20 billion and are among the 25 biggest banks in the country.

Analysts and bankers said all six companies placed on the watch list by Standard & Poor's -- including Allied Bancshares and Texas American Bancshares -- are likely to report depressed earnings and higher levels of loan losses, but said none of them is in serious difficulty.

First City Bank, the Houston bank that is the head of a chain of about 70 Texas banks, is generally considered to be the Texas institution with the most intractable problems because of its heavy loans to the Texas oil industry and its exposure in the depressed Houston real estate market.

First City has steadily lowered its dividend and its earnings have been based increasingly on one-time gains, such as sales of assets, rather than on profits from operations, according to James McDermott, the head of research at the securities firm Keefe, Bruyette & Woods -- a New York firm that specializes in analyzing bank securities.

First City was not among the banks named by Standard & Poor's yesterday.

Standard & Poor's said the recent drop in oil prices "will exacerbate existing loan problems caused by the 1983 energy recession." It noted that the six banks performed poorly in 1985, when oil averaged $25 a barrel. Although lower interest rates will reduce the cost of funds to the banks, their "performances will continue to be below historical levels."

Standard & Poor's generally places on its watch-list companies whose situations it is reevaluating. Standard & Poor's is one of the major bond-rating firms. These firms assign quality ratings to companies that issue debt securities, such as bonds. Generally, companies with higher bond ratings find it easier, and cheaper, to raise money than do companies with lower bond ratings.

In some cases, big investors such as pension funds and insurance companies will not buy bonds issued by companies with low bond ratings because repayment appears too risky.

William M. Isaac, the former chairman of the Federal Deposit Insurance Corp., said that the banks named by Standard & Poor's are reasonably strong, although he said it is reasonable to reasses the impact of declining oil prices and real estate problems on Texas banks.

"In my judgment, a herd instinct is developing and people are overreacting to energy and real estate problems," said Isaac, now head of the Secura Group, a bank consulting firm that specializes in assisting troubled banks.

Because of the steep decline in oil prices, a large number of oil exploration and drilling companies -- as well as the companies that sell them everything from meals to piping -- are in trouble. Big oil companies have been cutting back on the amount of money they are spending to search for oil because of the fall in the price of petroleum. Spot oil prices have fallen as much as 50 percent in less than three months.

Although the recent tumble in oil prices has produced a crisis in the exploration portion of the oil industry, the overall energy economy has been stagnant for several years. As a result, other industries that are dependent on oil -- from real estate to retail -- have been hurt.

Texas Commerce, based in Houston, reported yesterday it had to write off about $45.1 million in loans made to directors and expects to be unable to collect another $73 million in such loans. Texas Commerce, with about $20 billion in assets, said the directors were not granted any special treatment and said the Office of the Comptroller of the Currency, which regulates national banks, had investigated and found no violation of the law.