The largest apartment sale in U.S. history was announced yesterday when insurer MetLife (NYSE: MET) sold a Manhattan apartment complex for a whopping $5.4 billion.

Manhattan-based Tishman Speyer, a private owner/developer, bought the Peter Stuyvesant Town and Peter Cooper Village apartment complexes in a joint venture with BlackRock Realty. BlackRock is the real estate arm of money manager BlackRock Inc. (NYSE: BLK)

The twin complexes total more than 11,000 units, making them Manhattan’s largest single pair of apartment complexes. The property extends over 80 acres of land between east 23rd street and 14th street. The properties’ eastern boundary is FDR Drive, which runs north/south along the edge of the East River.

MetLife developed the properties after WWII as an affordable redoubt for veterans returning from the war. Both complexes went on to represent one of the few middle-income housing options in a city defined by extremely high housing costs.

Investment brokers accurately projected in recent weeks that heavy buyer demand could push the sale price well beyond the $4 billion mark. Given the lofty price tag, it’s unclear how Tishman Speyer will proceed: Roughly 75% of the units are rent stabilized, meaning they rent at below-market rates.

New York State laws could forbid Tishman Speyer from moving rent stabilized tenants out wholesale as a way to free up market-rate rents on their units. Many market watchers, who believe that the new owner must somehow justify this steep going-in price, expect Tishman Speyer to convert many of the units into expensive condominiums. Published reports speculating on the going-in capitalization rate, or initial yield, vary from 4.3% to as low as 3% on this deal.

The sale is expected to close in the fourth quarter. MetLife stands to make a $3 billion gain on the sale net of income taxes.