"Oh, that's grim. Yes, that's very grim."

That was the reaction of a New York art expert yesterday when he found out that three Jackson Pollock paintings had been stolen Thursday afternoon from the Cambridge apartment of Reginald R. Isaacs, Norton Professor of Regional Planning.

No one knew the precise value of the three works, but the best estimates pegged the total at $500,000. The value of a Pollock has skyrocketed in recent months after the $2 million sale of Pollock's "Blue Poles" to the Australian National Museum.

Isaacs acquired the paintings over a six-year period. He bought his first Pollock in 1946, on the same day that he met the American expressionist.

Isaacs said that both he and Pollock were "young and starving," and he bought the first painting by putting $10 down and paying $10 a month.

The three paintings stolen were:

* "no. 7, 1951", the most valuable painting, from Pollock's black and white period;

* an unnamed 1948 work, from his drip period; and,

* an unnamed 1949 painting, also from Pollock's drip period.

Pollock inscribed the 1949 painting "to Reg and family."

Both the Cambridge police and the FBI are investigating the theft. Cambridge detectives James A. Roscoe and Fidele Centrella met with Isaacs and his wife Charlotte for four hours yesterday afternoon. The two detectives solved another Harvard theft last May when they recovered $90,000 in stolen coins.

It seems that the heist was mastermined by at least two professional art thieves. The police speculate that two persons were needed to complete the crime because of the size of the paintings.

"no. 7, 1951" is 66 inches wide by 55 inches high. The other two works are both 30 inches by 36 inches

The theft took place between 3:15 p.m. and 4:45 p.m. Isaacs had returned home from the University at 3 p.m., quickly changed into his rowing outfit, and left for his daily workout on the Charles River.

When he returned from the boat house, he discovered the paintings gone and a briefcase left in the living room

Seeran Wisner, the day guard at the front security desk, said yesterday no unfamiliar persons left by the front door between 3 p.m. and 5 p.m. Police said yesterday the thieves probably exited by the fire escape.