CHICAGO, DEC. 29 -- Apex Holding Co., believed to be one of the nation's 10 largest privately held firms, has averted an auction of its vast corporate assets by filing for Chapter 11 protection from creditors.

Among the 83 Apex-owned companies that were to be sold in an auction today was Clark Oil & Refining Corp., which has 952 gasoline stations in the Midwest and Appalachia.

Chicago financiers Samuel Zell and Richard Lurie had agreed to buy Clark Oil last summer after Getty Petroleum Co. backed out of paying $447 million for the firm. But the deal, which Apex had told employes would close Dec. 1, has not gone through yet.

Apex, based in the St. Louis suburb of Clayton, Mo., filed for protection under the U.S. Bankruptcy Code last Thursday, a day after a bank declared the company delinquent on loans totaling more than $533 million.

To collect the money, the lender, Centerre Bank, had scheduled an auction for today at its St. Louis headquarters to sell off stock in 83 concerns that Apex had used as collateral for the loans.

Apex, which is extremely secretive in its dealings, said in a statement that the threatened liquidation "left us no choice" but to file for bankruptcy, protecting its assets until it draws up a reorganization plan to pay off its debt.

The Apex statement said the company had hoped to restructure its loans with the money from the sale of Clark Oil. But "with the recent drop in oil prices, Apex's lenders were unwilling to continue to fund the operations of Apex and Clark," the statement said.

Apex also complained that the banks tried to force it to sell Clark Oil, even cutting off cash from Clark customers, despite the fact that falling oil prices made this "an inopportune time to sell assets."

"Chapter 11 was our only realistic alternative," the statement said.

According to a banking source, Apex is controlled by Samuel Goldstein and Tony Novelly, both of the St. Louis area.

Apex bought Clark Oil for $478 million in 1981, just as oil prices topped out at three times their current level. The price drop has subsequently devalued oil properties, including Clark.

It is understood that Zell and Lurie were offering less than the $447 million Getty Petroleum had bid in June for essentially all of Clark Oil's assets, except its refinery in Louisiana.

Apex officials would not comment beyond the statement. Centerre also declined to comment, and Zell was said to be unavailable.

Apex has about 9,000 employes, and has been reported to be the largest privately held firm in Missouri and one of the top 10 in the United States. A source said Apex has had gross sales of $5 billion to $6 billion a year in the last 10 years.

Goldstein and Novelly got into the oil trading business shortly after the 1973 Arab oil embargo and quickly expanded in the oil industry and then into real estate, the source said, adding that the ventures were very profitable.

Apex's mistake, said the source, was buying Clark Oil at the wrong time.

Industry analysts say the company is probably marginally profitable at best.