By Chris Michaud

NEW YORK, Nov 11 (Reuters) - A Mark Rothko abstract sold for $45 million Tuesday at Sotheby's $344 million sale of contemporary and post-war art, which also set a record for Jasper Johns when one of the artist's seminal "Flag" works soared to $36 million.

Despite the hefty total, toward the low end of the $323-418.5 million pre-sale estimate for 78 lots, spending was restrained for the top works including the Rothko, which Sotheby's had estimated would sell near $50 million. Some top-priced works failed to sell.

The high point, while not the top price, was Johns's "Flag" from 1983. Estimated at $15-20 million, its $36 million easily eclipsed the artist's $28.6 million record.

Gerhard Richter's "Abstraktes Bild," with the same estimate, went for $21.4 million, while an untitled work by Robert Ryman set an artist's record when it sold for $15 million, the low estimate.

Other highlights included Andy Warhol's "Liz #3," a 1963 portrait of screen siren Elizabeth Taylor, which fetched $31.5 million against an estimate of about $30 million, the highest price ever paid for a Warhol portrait of Taylor.

Rothko's "No. 21 (Red, Brown, Black and Orange)," the top-estimated work, fell short with a final price of $45 million. Estimates do not include commission of just over 12 percent.

Alex Rotter, Sotheby's worldwide head of contemporary art, said the price did not disappoint and the estimate was between $35 million and $50 million - though Sotheby's own news releases listed it "in the region of $50 million."

With only 11 of 78 works unsold, he characterized it as "a strong sale."

"Moon (Yellow)," by Jeff Koons whose sculptures have drawn record prices in recent years including for a work by a living artist, went unsold when bidding ended at $11.5 million.

Another casualty was Warhol's "Little Electric Chair." It failed to sell with an estimate of $7.5 million to $9.5 million.

The uneven result contrasted with Sotheby's $159 million Monday sale of works from the estate of philanthropist Mrs. Paul Mellon, which far exceeded the high estimate with 100 percent of the offerings sold.

The focus now turns to Christie's, which set records twice in the past year for the biggest auctions in history as it holds its post-war and contemporary art sale on Wednesday, carrying a pre-sale estimate of more than $600 million.

(Reporting by Chris Michaud; Editing by Ken Wills)