"PLEASE KEEP THIS DOOR CLOSED. MOLD (TYPE A) GROWING INSIDE. DO NOT OCCUPY!"

That sign isn't a prop from a Grade B Hollywood science fiction movie. Rather it is a warning posted (with editorial comments added by workers) on the door of an office inside the plush-appearing Columbia Plaza building. Uncle Sam rents the 12-story structure from the government of Kuwait. The rent is $2.3 million a year.

For the past few months the building, near the Watergate complex, has been plagued by water leakage, falling tiles and mysterious fast-growing greenish black mold. The mold, on doors, walls, chairs, typewriters and telephones, has made some offices unusable. Some of the 1,000 federal workers there claim it had made them ill.

The building, originally intended to be an in-town motel, is leased by the General Services Administration. A Washington firm manages the property for its principal owners, the Persian Gulf shiekdom of Kuwait.

Columbia Plaza is home for the headquarters of the Equal Employment Opportunity Commission, and also houses some operations of the Bureau of Mines, the State Department and some hush-hush Pentagon intelligence activities.

Earlier this year some workers at Columbia Plaza complained of headaches, sore throats and dizzy spells. This was traced to carbon monoxide fumes - which were found to be at "tolerable" levels according to government health experts - seeping up from underground garages. That problem had been corrected, but then recently the mold appeared, apparently fostered by dampness from water seeping into the building from the fifth and third floors. GSA is in the process of letting a contract to replace 110 windows in the building where water is leaking in.

Now, along with the mold, the ventilation "imbalance" has returned as ceiling tiles get wet and crash down in offices and halls, opening new passages for both water and auto exhaust. Water damage in some of the lower floors has apparently ruined expensive carpeting, damaged some government files and forced some offices to be evacuated. Meantime, the mold grows.

GSA clean-up crews have been busy removing the growth-which gives many offices a strange, unpleasant smell - but it comes back. Recently, GSA issued a memo to occupants of the building on methods to "combat the mold, which is proliferating at an incredible rate . . . " They include leaving window blinds open night and day to admit light, and keeping lights burning 24-hours a day to kill the mold, which prospers in damp, dark conditions.

Top help bring the mold under control, the government is planning to buy and install 25 large dehumidifiers.

Workers in the building have set up an unofficial "mold hot line." They advise each other of the advance of the gookie stuff and have established a ceiling watch to help catch falling tiles before they fall. Some sporting types have even worked up a pool betting when and where the next outbreak will occur.

Temporary repairs are being made, with tar and tar paper, on the third and fifth floor outside malls. This is where most of the water collects and where rain leakage to lower floors is the biggest problems.

Officials say no health problems have been identified. But some employes on lower levels of the building say the mold has produced a rash of illnesses, absenteeism, and a rush to doctors for allegy tests.

Officials hope various corrective measures will have the model problem under control by the end of this year.

Columbia Plaza was built 10 years ago on land that once contained rundown row houses before an urban renewal plan for the area was conceived. Most of the property was formerly owned by parking magnate and developer Dominic F. Antonelli Jr. and his partner, Kingdon Gould Jr., and the two men had substantial interests in the Columbia Plaza complex before it was sold to its current owners about a year ago.

Anne Wexier, special assistant to President Carter, is the luncheon speaker Friday at the American Society for Public Administration's George Washington U. meeting. Call 377-5930 for details.