AHITHERTO unknown painting said to be by one of the America's most famous 20th-century artists -- the late abstract expressionist Mark Rothko -- has turned up recently under "Antiques and Collectibles" in the classified ads.

The ad, which appeared in The Washington Post and The New York Times, was placed by Gailey Smith of Tyron, N.C., widow of well-known Washington journalist Merriman Smith. Los Angeles dealer Ron Rakow saw the ad, contacted Smith, and has joined with her as "joint venturer" in the disposition of the painting.

The 4-foot-by-5-foot horizontal canvas, an irregular vertical stripe in magneta, pink and apple green, is said to have been painted in 1946, while Rothko was still painting surrealist abstractions. The work has yet to be authenticated, but several Rothko experts have expressed bafflement upon seeing a color photograph of the painting, which they all agree is atypical of the artist's work. "It's an oddball Rothko," admits Gailey Smith. Though her original asking price was $150,000, the ad said, "make an offer."

"Mark Rothko was a friend of my mother's, and I got the painting in a most unusual way," Smith explained by phone from her North Carolina home. "In 1946, when I was 13, I and my governess and my mother went up to Santa Barbara from Palm Springs to see the uncrating of a Rothko show in the Santa Barbara Museum -- all typical surrealist works he was doing at the time. Rothko was there, and when they uncrated this one -- a hot pink, magenta and apple green vertical stripe -- he laughed and said, 'Look what they sent! It's my experiment!'

"I was feeling very rich because I had $264 in the bank left over from a birthday present, and when Rothko insisted they send the painting back, I offered to buy it. I loved it because it was my favorite color, pink. He said he wouldn't sell it, but asked, 'Can I give it to your for a belated birthday present?' I was thrilled beyond belief, and took it.

"Two years later, Rothko visited our home, and I asked him, 'If you're ever famous, how will anybody know you did this painting?' I was stupid, of course, and didn't know he was already famous. But he took my mother's sumi brush and scrawled a big "R" in the lower right-hand corner.

"Year later I was working in New York for the Theater Guild, not far from his studio on West 53rd Street, and we renewed our acquaintance. In 1960, after I married Merriman Smith, [Pulitzer Prize-winning UPI White House reporter] and moved to Washington, we met again when Rothko came to hang two paintings the Phillips Collection had just bought. He and my husband became good buddies -- both were chain smokers and loved to drink -- and they'd go off for three days and have a grand old time. We saw each other often in New York until Rothko committed suicide Fed. 25, 1970. My husband committed suicide April 13, 1970. Mark's death had affected him deeply."

After her husband's death, Smith lived in Florida, where she remained until last fall, when she moved to Tyron, home of her godfather, Philip Wylie, author of "Generation of Vipers." "I wouldn't be selling the painting now, but I fell in love with a house," she said, "and after 17 years in probate, it's coming up for sale. I looked at the painting and sort of asked Mark, 'If you were me, would you trade it for the house?' I know it sounds nutty, but he was a great house lover, and I know he understands."

Asked why she chose to sell the painting through a classified ad, Smith said, I've never done this before, and needed cash fast. This seemed like the best way to get it."

Authenticating the work may not be easy, and it has already met with guarded interest and some skepticism on the part of the New York Rothko establishment. Said Joyce Schwartz at Pace Gallery in New York, Rothko's primary dealers, "It sounds very strange, and without seeing it I can't say anything about its value or authenticity. We have a show right now of paintings from those years, 1943-46, and they're all surrealist oils and watercolors." Jeffery Hoffeld, also of Pace, was similarily skeptical: "I can't give you any information without seeing the painting, but don't hold your breath."

The only Rothko authority who has seen the painting since Rakow had it shipped to New York is Bonnie Clearwater, curator of the Rothko Foundation, who is compiling a catalogue raisonne on Rothko. "It's not like any other Rothko I've ever seen, and the signiture is most unusual," said Clearwater, "but then I don't make authentications. For that they'll have to got to a dealer or to a museum curator who is an expert on Rothko."

Clearwater did say, however, that "late 1946 to '49 was an experimental period for Rothko, a time when he started to loosen up and move away from surrealism into works called 'multiforms,' before he began to develop his mature style. If this is a Rothko, it would fit into that period, and from the scholarly point of view, could be an important painting."

New York dealer-collector and Rothko expert Ben Heller said, "Rakow showed me the photograph, but I haven't seen the painting yet, so at the moment all I can say is that it may be a Rothko, I don't know. It certainly doesn't look like a Rothko, but if it is an experiment, and what the owner says is true, it would be an important addition to what we know abuout the artist."

Meanwhile, dealer Rakow, who was in New York when he saw the ad and had the painting shipped to him there, has now returned to Los Angeles and is pursuing the authentication of Smith's story on his own. He said he has already located her mother's former secretary, Marion Toole, and plans to get from her a sworn affidavit saying that she recalls the day when Rothko gave the painting to Gailey Smith.

"I like her and I believe her," says Rakow, who managed "The Grateful Dead" before going into the art business, and says he has found other treasures in the classified ads, including 400 paintings by a little-known German-born Chicago artist named Julius Moessel. Two scholars are currently writing a catalogue raisonne of his work.

Rakow says he may not end up putting the Rothko on the market, but may instead sell it someone who will donate it to a museum and take a tax deduction. "I think the Rothko should go to a museum as an important evolutionary piece, as an experiment that gives new insight into an important period that led to his mature work. It should go to an organizaton whose function it is to edify." He said he may make the gift himself.

"But first I'm trying to satisfy myself that I have a Rothko," says Rakow. "Then I'll go to the experts, of which there are more than just a few people in New York."

Meanwhile, Smith 48, says, "I'm happy to let Mr. Rakow do the legwork. Let's face it: Tryon, N.C., is not a world art center. I anticipate being alive for years but on the other hand, I think it's important for me to establish the validity of the painting while I'm still around. After all, with my mother ded, and my governess dead, and my husband dead, and Mark dead, I'm in a fix."