Continental Illinois Corp., the nation's biggest business lender, yesterday reported a loss of $61 million in the second quarter because of the hundreds of millions of dollars of bad loans it bought from the now-defunct Penn Square National Bank.

It was the biggest loss ever recorded by one of the 10 giant U.S. money-center banks, according to James Wooden, vice president and banking specialist for Merrill Lynch, Pierce, Fenner & Smith. In fact, until Chase Manhattan Corp. reported a second-quarter loss of $16.1 million on Tuesday, analysts could not recall a major bank reporting a loss.

Continental, the nation's sixth-biggest bank company, bought more than $1 billion in energy loans from Penn Square. It estimated yesterday that $220 million of those loans will go bad. It already has written off $45 million and classified another $151 million as nonperforming assets.

Continental's $61 million loss compares with a profit of $58 million ($1.47 a share) in the second quarter of 1981. For the first half of the year, the bank earned about $11 million (27 cents) compared with $120 million ($3.04) last year.

The bank said that it wrote off $82.2 million in bad loans in the second quarter, including the $45.1 million for Penn Square loans. Its reserve for potential loan losses totaled $476.6 million on June 30.

Meanwhile, BankAmerica Corp., holding company for the nation's second-biggest bank in terms of assets, reported profits of $121.3 million (82 cents a share) in the second quarter, down 6.4 percent from $129.6 million (88 cents) in the second three months of 1981. President Samuel H. Armacost said the "troubled California real estate market" forced the bank to increase its reserve for potential loan losses by $100.5 million compared with $82.6 million last year.

Additions to loss reserves come directly out of a bank's profits. When a bank actually writes off a loan as a loss, it deducts the loss from the reserve, rather than from its current profits.

The July 5 failure of the $500-million-asset Penn Square bank in Oklahoma City, coupled with the $2 billion in loans it made and sold to other banks, rocked the financial system.

Chase Manhattan, the nation's third-biggest bank, reported that it had $45 million in bad loans from Penn Square. Other banks also were burned.

Chase's $16.1 million loss, however, was directly related to the failure of another Chase client, Drysdale Government Securities Inc. Chase lost $117 million after taxes when it had to pick up the debts of Drysdale.

Continental, which has become the biggest commercial lender in the country because of its aggressive lending policies during the last five years, is feeling the impact of the recession on its portfolio. The bank said that $1.3 billion of its $35 billion in loans are nonperforming--either loans that are past due or loans on which terms have been changed, or real estate the bank owns because of foreclosure. A year ago its nonperforming assets were $519 million, 1.7 percent of total loans. Continental recently foreclosed on the McLean Gardens condominium development in Northwest Washington.

Historically, most nonperforming assets do not end up as bad loans.