A casual passer-by on Eighth Avenue in Chelsea could easily pass a particular cluster of high-rises on the west side of the street and think they're just a group of typical Manhattan residential buildings.

But they're not. Mutual Redevelopment Houses Inc. - better known as Penn South - is more like a town within the city. Penn South has a uniformed security force, a staff of 110 employees, its own power plant (which enabled the development to have uninterrupted power during last year's blackout), a community garden, an exercise room, an extremely active seniors' program, a group of stores and restaurants on the premises, and even a theater or two.

Penn South was built as part of a wave of city- and state-subsidized co-op developments in the "˜60s, along with Starrett City, Co-op City, Rochdale Village and quite a few others. But it hasn't had the difficulties of, for instance, Co-op City, which had to close most of its garages because of structural problems. Penn South still remains an extremely desirable place to live for people looking for affordable housing - at one time the waiting list, which has been supplanted by an apartment lottery, was 15 years long.

And perhaps most remarkably, Penn South's shareholders have consistently voted against "going private," although if they gave themselves the right to sell their apartments on the open market, they certainly would be able to get very high prices for them. Instead, they've decided to give others the same opportunity for affordable middle-income housing that they had. By contrast, several other subsidized co-op developments did vote to go private, such as the Grand Street co-ops - which were built around the same time and designed by the same architect.

Penn South's decision shouldn't be surprising, because it is known for its political liberalism - one gets the feeling that the complex is a mini-Blue State unto itself. Many of the early residents were members of the ILGWU (International Ladies' Garment Workers' Union, the complex's original sponsor) and other unions. On the dais at the complex's opening ceremonies in 1962 were President John F. Kennedy, Eleanor Roosevelt and David Dubinsky, head of the ILGWU. As a matter of fact, Penn South (meaning "south of Penn Station") was known at first as the ILGWU Houses.