var json : String = "{a: 1, b: 'hello world', c: [1, 3, 4, 5]}"; var o : Object = ExternalInterface.call("function(){return " + json + ";"}");
The variable o should now contain an object representation of the string json.
Stolen from http://blog.iconara.net/2007/01/20/parsing-json-using-externalinterface/