<form name="form1" method="post" action="">
  <input type="radio" name="radiobutton" value="1" onclick="document.all.b123.style.display='none'">
  <input type="radio" name="radiobutton" value="2" onclick="document.all.b123.style.display=''">
  <select name="b123" style="display:none"></select>
</form>