An unidentified man robbed a cashier yesterday morning at the Harvard Business School Cooperative Society at gunpoint, Harvard University police officials said yesterday.

The suspect, who wore a ski mask, approached the employee and told her to fill up his red bag with cash, according to police.

The cashier then asked if he was kidding.

"No, I'm not kidding," the man responded. He then pointed a silver handgun at the cashier, and she filled the bag with roughly $5500.

The man, described as tall and thin and in his twenties, then escaped in a maroon car parked near the Coop and drove in the direction of North Harvard St.

Harvard police officials called the crime a "big deal."

"This kind of thing never happens there," police said. "They don't even have shoplifters."

Police officials said the robbery was risky because that area of the Business School is well patrolled.

"The guy was pretty crazy for what he did," officials said.

In an unrelated incident, Harvard Policearrested two males for assault and battery on apolice officer.

Robert M. Farrell was spotted throwing a brickthrough a car window at 12:40 a.m. yesterday byOfficer Dennis Maloney.

Maloney chased the suspect down Bow St., wherea struggle ensued, Police Chief Paul E. Johnsonsaid.

Additional officers from Harvard and Cambridgeassisted in the arrest. But when they attempted tohandcuff the suspect, John Farrell, Robert'sbrother, spit into the face and chest of one ofthe arresting officers.

After a short scuffle, John was also arrested,Johnson said. Following a short body search,police officers found maarijuana on Robert's body.

Both were charged with malicious destruction ofproperty and assault and battery on a policeofficer.

The two brothers were arraigned this morning ina Cambridge district court