Mary Treadwell, facing a rigorous and acerbic four-hour cross-examination at her fraud and conspiracy trial, acknowledged yesterday that she forfeited more than $4,000 in tenants' security deposits and money in an employes' Social Security tax account to pay off a bank loan to one of her firms.

Moreover, Treadwell testified that it did not disturb her that P.I. Properties Inc., the now-defunct real estate firm she headed in the mid-1970s, illegally used the security deposits of the impoverished tenants at Clifton Terrace, which the firm owned, as collateral for two loans to another Treadwell-run company.

Assistant U.S. Attorney William D. Pease and Treadwell engaged in a series of icy exchanges during the cross-examination. But upon hearing that Treadwell was not upset by the use of the security deposits as loan collateral, the prosecutor shouted at her, "Did you ask the tenants if you could use their security deposits as collateral?"

"Mr. Pease, I never put my problems on the shoulders of others, including the residents . . . ," Treadwell replied. "I went and I did it and I take responsibility for it."

But the 42-year-old Treadwell made few other concessions to Pease, despite the assertions in his questions that at times seemed designed mostly to spell out the government's view of the case to the federal court jury of eight women and four men.

Treadwell traded barb for barb with Pease, telling him at one point, "I've only learned about these transactions after I got a lawyer and the resources you've had for four years."

In another exchange, when Pease asked if Treadwell could "safely assume" a conclusion he had drawn, Treadwell shot back, "After five years in this case, no sir, I would not assume a thing."

Treadwell is accused of using P.I. Properties to defraud the federal government and Clifton Terrace's tenants of thousands of dollars to enrich herself.

In addition to fraud and conspiracy, she is charged with tax evasion and making false statements to federal officials.

While acknowledging numerous improprieties in the operation of P.I. Properties, she has largely placed the blame for any wrongdoing on the firm's former general manager, Robert E. Lee, and her sister, Joan M. Booth, Clifton Terrace's one-time project manager. Lee and Booth have already pleaded guilty to conspiracy and other charges in the case.

Treadwell said that the forfeited security deposits were those of tenants at the Buena Vista and Kenesaw apartment buildings, two other projects that P.I. Properties managed. The forfeiture, she said, was made to pay off part of a $19,000 loan that she only belatedly found out Lee had secured on behalf of P.I. Properties without her authorization.

"The tenants of Clifton Terrace were not paying for my personal expenses," she told Pease. Later, she said that "the only thing I did do wrong and that doesn't involve Clifton Terrace" was to not file her 1976-78 federal income tax returns until 1980.

The government has alleged that she evaded more than $33,000 in taxes for the three years.

Treadwell headed several companies that were an outgrowth of the 1970s job-training program known as Youth Pride. As a result, Pease repeatedly tried get her to take responsibility for the actions of the people she hired.

But when asked about one transaction by Booth, Treadwell said simply, "No, I did not know what my sister was doing."

Treadwell added that the strain of the case had created "a very wide family split." She said that she and Booth lived together for awhile in the late 1970s, but "the only way we could live under the same roof was not to talk about Clifton Terrace."

When Pease finished his cross-examination, John W. Nields, one of Treadwell's court-appointed attorneys, asked her whether she had made any mistakes in owning and managing Clifton Terrace.

"Of course I did," Treadwell replied. "Right off the bat I made a mistake in who my management team was."

Nonetheless, Treadwell said that she would again purchase the property if she had the chance.

Treadwell testified for more than 13 hours over four days and when it was completed, she appeared to be choked with emotion.

She sat quietly at her lawyers' table and cried as the jury left the courtroom for the three-day holiday weekend.