John A. Zaccaro, husband of Democratic vice-presidential nominee Geraldine A. Ferraro, was involved in two multimillion-dollar real estate transactions last year that now are being investigated by a Manhattan grand jury.

It is not clear whether Zaccaro is a target of the investigation, and his criminal attorney, Daniel Beller, has refused to comment on the case. District Attorney Robert Morgenthau also has declined to comment, as have most of the other individuals involved.

But a source familiar with the case and with associates of Zaccaro said the inquiry seems to include questions about Zaccaro's business relations with three men, Harold Farrell and Robert Wolk, who worked out of Zaccaro's office last year, and Ronald Harnisch, who worked with Ferraro in the Queens district attorney's office in the mid-1970s.

Farrell was an attorney until he was disbarred in 1966 for bribing an official to obtain state liquor licenses. Wolk and his brokerage firm were barred from doing business by the Securities and Exchange Commission in 1981 after irregularities were found on a prospectus. According to associates of Zaccaro, he wasn't aware of their background and asked them to leave his office late last year.

The first transaction involved a $550,000 loan in September 1983 to a Zaccaro client from a credit union for employes of the local port authority, where Harnisch was counsel. Federal auditors found last spring that the loan was improper because the recipient, John DeLorenzo, was not a member of the credit union.

DeLorenzo used the loan in purchasing a $5 million apartment house in Manhattan. Zaccaro and Harnisch were supposed to split a $100,000 commission on the sale.

One official familiar with the transaction said the loan was approved by only three of the credit union's nine board members and included a $475,000 check made out to Delorenzo, a $53,000 check made out to Farrell and $22,000 for prepaid interest.

The New York Times quoted Farrell last month as saying Zaccaro had directed him to put the $53,000 in his personal bank account and write checks totaling $32,500 back to Harnisch and one of the directors who approved the loan. Harnisch and the director are said to be cooperating with investigators.

Farrell and his attorney would not discuss the case, and it could not be learned whether Farrell has given investigators his account of Zaccaro's alleged instructions.

A Zaccaro associate, who asked not to be quoted by name, said Zaccaro didn't play any role in arranging the credit union loan.

Anthony Essaye, an attorney for the Mondale-Ferraro campaign, said Farrell's credibility should be questioned because of the bribery that led to his disbarment.

"While I cannot discuss the details of the matter, the essence of it is that Farrell is an effective con man and unfortunately John Zaccaro was one of many people who have been conned by him," Essaye said.

The second transaction involves alleged falsification of a $12 million loan application on another DeLorenzo deal last year. Some of the details came out in a civil suit against DeLorenzo by a corporation whose stockholders were reported to be Zaccaro, Farrell and Wolk.

Records in the suit show that Zaccaro stood to share in a $1 million commission and a 25 percent interest in the building if he and his partners secured financing for DeLorenzo and won the suit. A Zaccaro associate said Zaccaro told Farrell and Wolk that he wanted no part of that commission or ownership interest. Zaccaro sided with DeLorenzo in the suit against his reported partners.

In his affidavit in the lawsuit, DeLorenzo accused Farrell of "fraudulently" changing the sales price in his contract from $12 million to $15.5 million.

The money was to buy a 500-unit apartment complex in Queens, but the deal fell through after the allegedly falsified loan application. Zaccaro has said he didn't know anything about the alleged changing of the contract price.