The Dresdner Bank of Frankfurt was the largest bidder yesterday at the second U.S. gold auction this year. The Treasury realized $56.1 million from the sale of $300,000 ounces at an average price of $187.06.

The West German bank submitted bids for 224,000 ounces of gold at prices ranging from $186.87 to $187.06 an ounce. Successful bids ranged from a low of $186.52, or 2 cents above yesterday's afternoon London gold fix, to $190.29.

The government intends to auction 300,000 ounces monthly for six months to bolster the dollar by depressing the price of gold. At the May sale - the first in three years - demand was heavier, as nearly twice as many bidders signaled their intention to buy 1.3 million ounces. The average price then was $180.38.

Yesterday the General Services Administration, which conducts the auction, received bids from 31 individuals or companies for 1 million ounces. Twelve million, seven hundred thousand dollars from gross receipts will be used to retire gold certificates, and the remainder will go into the Treasury.

Treasury lawyers declined to name the 21 successful bidders and the amounts purchased yesterday until bidders had been officially notified of their acceptance. However, a GSA spokesman furnished the names of those who had submitted bids at prices above the accepted minimum of $186.52. These included the Swiss Bank Corp. of Zurich, Johnson Mathey Co. Ltd. of London, Monex International of California, Union Bank of Switzerland, Credit Suisse of Zurich and several New York metals companies.

The next Treasury gold auction is scheduled for July 18.