IN THE KEYS, key limes are used for more than pies. There are key lime realtors, key lime postcards, key lime jelly beans, key lime fudge, even key lime honey. For the locals lucky enough to have a tree, key limes can be used for marinating meat, garnishing platters or squeezing for juice. But it's the pie -- that simple mixture of lime juice, egg yolks and sweetened condensed milk -- that put the key lime on the map.

The recipe developed when refrigeration was scarce in the secluded Keys. Luckily, there was a substitute for fresh milk -- sweetened condensed milk, which does not require refrigeration. It's still used, even though a custard filling of heavy cream mixed with juice produces a creamier, richer pie.

Pursuing the ideal Keys key lime pie may be a fruitless search. But there are some good ones to be found. Although the recipe is standard, the differences in Key pies are surprising. A primary factor is the type of juice used -- reconstituted persian lime, bottled key lime, fresh persian, or in a few cases, fresh key lime. And then the number of egg yolks. One restaurateur said many places even omit the egg yolks to cut costs. Then there are the crusts -- a standard-dough crust as opposed to graham cracker. And then the topping, meringue versus whipped cream. But, according to one old-timer waitress, there are differences simply because, "Some people just don't know how to cook."

Whatever, here is a guide to the pies. Three stars for the best, on down to the one star (the lemons).

* * * Cheeca Lodge, Islamorada -- Key lime trees aside the driveway omened well. Besides a tart, unartificial lime flavor, this authentic pie had a good graham cracker crust and fine whipped cream to match.

* * * Half-Shell Raw Bar, Key West -- The chartreuse-colored filling may have been intimidating, but this was one tasty pie. Properly tart, firm-crusted and admittedly made with real persian limes.

* * 1/2 Joe's Stone Crabs, Miami Beach -- A pie for all tastes; not too tart, not too sweet. Well-made crust and whipped cream.

* * 1/2 Manny and Isa's, Islamorada -- Here the waitress gives you a pie and a story: Rumor has it that Fern Butters, who lived to 90, used to own the restaurant, then called The Fern Inn, and there invented the key lime pie. It's not hard to believe, given the heavy-duty tartness of her successor's pie. Real key limes may make you wince, but with good ingredients and a substantial head of meringue to soften the bite, this pie is easy to get used to. Manny has a large grove of key limes in his yard (about 200), squeezes the juice and freezes it for pies to come.

* * Claire's, Key West -- Grainy texture, but none of the canned-milk taste many of the others had. Top-notch freshly whipped cream.

* * Lorelei, Islamorada -- Very custardy and very tart. Dark green flecks suggest persian influence, restaurant says otherwise.

* * Mac's Barbecue, Key Largo -- A diner-turned-bakery where the 25 pies sold daily are, according to the waitress, made by "Mrs. Mac." Nice lime flavor, decorative meringue. Overall taste good but not distinctive.

* * Mrs. Biddle's, Key West -- She bottles and sells her own persian lime juice for pies, in addition to making key lime fudge, jelly beans and jam. Stick with the pie.

* 1/2 Pier House, Key West -- Meringue piled bouffant-style tasted in need of a rinsing, soapy-flavored as it was. Lime flavor a touch too sweet.

* Captain Bob's, Key West -- Watery filling spoils a good, firm crust.

* The Deli, Key West -- A poor combination of soggy crust and powdery filling.

* Green Turtle Inn, Islamorada -- This is the restaurant that runs the cannery. Enough said.

* Marker 88, Islamorada -- Better to skip the grainy pie and try the Key Lime Baked Alaska -- a cool lime sherbet covered by a marshmallowy film.

* Mike Gordon's, Miami Beach -- Another grainy one.

* Port of Call, Key West -- Starchy and pasty, a reminder of how instant key lime pie might taste.

* Rod's, Key West -- A recommendation from more than one local; but it was an overly gelatinous filling in a crumbly thin crust.

1/2* Townsite Supermarket, Islamorada -- "Famous key lime pie sold here," boasted the banner. Famous for being the worst, maybe. The cheapest pie in all the Keys (99 cents), but you're better off spending 10 cents apiece when the store stocks fresh key limes, packing them on your carry-on luggage, and being the only Washingtonian to make real key lime pie.

ORIGINAL KEY LIME PIE (Makes a 9-inch pie)

This is the age-old recipe. 4 eggs, separated 1/2 cup fresh lime juice 14-ounce can sweetened condensed milk 9-inch baked pie shell, graham cracker or dough

For the meringue: 4 egg whites 1/2 teaspoon cream of tartar 1/2 cup sugar, or to taste

Beat egg yolks until light and thick. Blend in lime juice, then milk, stirring until mixture thickens. Pour into baked pie shell.

If you are making the meringue, beat egg whites with cream of tartar until stiff. Gradually beat in sugar, beating until glossy peaks form. Spread meringue over surface of pie to edge of crust. Bake in 350-degree oven until meringue turns golden brown, about 15 minutes. Chill before serving.

For an alternative topping, use 1 cup heavy cream, whipped. This topping does not require that you cook the pie; just spread the whipped cream over surface of pie and refrigerate before serving.

PURISTS' KEY LIME PIE (Makes a 9-inch pie)

This recipe omits the condensed milk and substitutes heavy cream instead. 1 cup sugar 1/4 cup flour 3 tablespoons cornstarch 1/4 teaspoon salt 2 cups whipping cream 3 eggs, separated 1 tablespoon butter 1/4 cup fresh lime juice Grated rind of 1 lime Baked 9-inch pie shell, graham cracker or dough

For the meringue: 3 egg whites 1/4 teaspoon cream of tartar 1/3 cup sugar, or to taste

In a saucepan, combine the sugar, flour, cornstarch and salt. Gradually stir in the cream, and heat, stirring constantly, until cream boils. Reduce heat to low and simmer a few minutes while you beat the egg yolks in a medium bowl. Gradually pour half the hot cream mixture into the egg yolks, stirring briskly. Return that mixture to the pot and, stirring, heat until thick, but do not let it boil. Remove from heat and add the butter, lime juice and rind. Stir and cool slightly. Pour into baked pastry shell and cool.

Top with a meringue made by beating the egg whites with cream of tartar until soft peaks form, then beating in the sugar until mixture is stiff and glossy. Cover with the meringue to the edge of the pie shell. Brown in a 350-degree oven for 15 minutes or until golden.

Or substitute whipped cream for the meringue and refrigerate immediately.