A major New York investment banking firm has been accused of improperly trading $11 billion worth of securities bought with public funds on behalf of the County of Los Angeles.

The firm, Donaldson, Lufkin & Jenrette, denies any impropriety. William Fisher, executive vice president of DLJ, said "the county had to know in excruciating detail what was going on."

The Los Angeles Times reported in Friday's paper that the third-ranking official in the county treasurer-tax collector's office is under investigation by federal securities regulators for his dealings with DLJ.

But the newspaper quoted Treasurer-Tax Collector H.B. Alvord as clearing the employe and laying the blame on the DLJ official on the account.

At issue is whether the DLJ executive, Douglas Robbins, a senior vice president, "churned" the account, earning huge commissions for himself and his firm by unnecessarily buying and selling Treasury securities and other financial instruments.

The county claims there were 2,200 transactions by DLJ between January 1981 and March 1982. By comparison, the county says it invests public funds through a dozen other banking firms besides DLJ and each does only six to 10 transactions a month.

Fisher said that each transaction was confirmed daily with the treasurer's office in Los Angeles.

But attorneys for the law firm Tuttle and Taylor, which has been retained by the county, said in a telephone interview that the county had been told by the firm to ignore confirmation statements.

"DLJ told county officials, 'You can ignore our monthly statements. Our computer is all screwed up'," said one attorney.

When the questionable transactions came to light recently, the county had $92 million in allegedly unauthorized investments in its name, according to the county, on which the county could face losses of $13.5 million because of declining market prices, according to attorneys at Tuttle and Taylor.

If the trades were not, in fact, ordered by the county, then there will be complex questions about DLJ's profits from the trades and the county's losses, which attorneys say were considerable during the 15 months.

The county has requested the Securities and Exchange Commission to investigate the affair.