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

}

function handleHttpResponse()
{
   campo_select = document.Bairro.Bairro;
   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.