"How could someone think that it is going tobenefit civilization by turning back the clock,"Herschbach said.

Hershchbach and other professors said they hadbeen checking their mail carefully, especiallysince the most recent string of letters.

Emery Professor of Organic Chemistry Elias J.Corey said yesterday he was watching out forsuspicious packages.

"My wife and I are being very careful, and mysecretary as well," he said.

Corey, who was awarded the Noble Prize forchemistry in 1990 for his work allowing scientiststo make complex new molecules form ordinarychemicals, said he thought it was appropriate thatthe University take some security precautions.

"I think we have to face reality that Harvardis a prime target," Corey said. "It's alreadyhappened at Yale."

One member of the Harvard Medical Faculty saidyesterday he had already begun checking all of hismail prior to the most recent mailings.

"If I see a package, and it doesn't have areturn address I would absolutely not open it," hesaid.

Rando said he called the Harvard police overthe winter break, when a package arrived from theBoston area with no return address.

"When you call the police, it is not an easysituation," Rando said yesterday. "They evacuatethe building, if they do it by the rules. You haveto go through a big hassle, it's not like theybring in a little machine and X-ray it."

The FBI has refused to release the contents ofRoberts' letter. Yesterday, his telephone numberwas changed to an unpublished number, theAssociated Press reported.

The letter was mailed to Roberts at work at NewEngland Biolabs in Beverly. The company is one ofmany trying to make a profit from the work inbiotechnology Roberts and Sharp pioneered.

FBI officials asked professors to be as carefulas possible in examining their mail.

Harvard police guidelines issued in the wake ofthe Oklahoma City bombing warn against thefollowing types of parcels:

. Foreign mail, air mail and special delivery