function list_dados( valor )
{
  http.open("GET", "result_cidade.php?id=" + valor, true);
  http.onreadystatechange = handleHttpResponse;
  http.send(null);
}

function handleHttpResponse()
{
  campo_select = document.form1.cidade;
  if (http.readyState == 4) {
    campo_select.options.length = 0;
    results = http.responseText.split(",");
    for( i = 0; i < results.length; i++ )
    { 
      string = results[i].split( "|" );
      campo_select.options[i] = new Option( string[0], string[1] );
    }
  }
}
function getHTTPObject() {
var req;

try {
 if (window.XMLHttpRequest) {
  req = new XMLHttpRequest();

  if (req.readyState == null) {
   req.readyState = 1;
   req.addEventListener("load", function () {
   req.readyState = 4;

   if (typeof req.onReadyStateChange == "function")
    req.onReadyStateChange();
   }, false);
  }

  return req;
 }

 if (window.ActiveXObject) {
  var prefixes = ["MSXML2", "Microsoft", "MSXML", "MSXML3"];

  for (var i = 0; i < prefixes.length; i++) {
   try {
    req = new ActiveXObject(prefixes[i] + ".XmlHttp");
    return req;
   } catch (ex) {};

  }
 }
} catch (ex) {}

alert("XmlHttp Objects not supported by client browser");
}
var http = getHTTPObject();
// Logo após fazer a verificação, é chamada a função e passada 
// o valor à variável global http.