Merrill Lynch Canada Inc. said today that thieves who intercepted two of the company's messengers Friday got away with about $51.3 million worth of securities in one of the largest heists in history.

The stolen goods included $30.3 million worth of negotiable securities, $21 million in non-negotiable securities and a few thousand dollars in cash, said company spokesman James Coutts.

However, he said the investment firm had issued what amounted to stop-payment orders on the negotiable securities, which included Canada Savings Bonds.

"We've distributed serial numbers to transfer agencies and the banks," Coutts said. "The negotiable securities are not going to be very useful to the thieves. They won't get cash for them, no matter what they do."

Coutts said it might be possible for the thieves to find some other use for the negotiable securities, but "I'm not going to get into that at all."

The company has posted a "substantial" reward for information leading to the arrest of the three gunmen who held up two Merrill Lynch messengers as they were transferring the securities in bags and strongboxes from a bank to the company's downtown Montreal office.

The exact amount of the reward was not released at the request of Montreal police, Coutts said.

Company staff worked through the weekend to determine which securities had been stolen. To prevent the securities from being cashed, they also traced the serial numbers and searched through thousands of documents between the firm and its myriad of transfer agents.

Coutts said the work should be completed this afternoon.

"We're the second-largest firm of its kind in the world," he said. "Anyone with our level of sophistication should be able to handle it smoothly."

Coutts said all the securities were insured and replacement certificates would be issued to Merrill Lynch customers.

The thieves struck at about 9:30 a.m. EST Friday as the couriers carried the metal securities box through the lobby of the office building where Merrill Lynch has its corporate office.

The gunmen forced the couriers to a basement washroom at gunpoint, then fled with the box of securities.

Merrill Lynch, with 520 offices in 20 countries, carries out billions of dollars worth of business each year.

The largest robbery in postwar history occurred Nov. 26, 1983, at Britain's Heathrow Airport when masked gunmen stole $39 million in gold -- 6,800 ingots weighing three tons.