<html>
<head>
<title>Documento sin título</title>
<script language="javascript">
function Bloquear(){
if(document.form1.chkcli.checked){
document.form1.clientes.disabled=true;
}else{
document.form1.clientes.disabled=false;
}
}
</script>
</head>
<body>
<form id="form1" name="form1" method="post" action="">
<p>
<input type="checkbox" name="chkcli" value="checkbox" onclick="Bloquear();"/ checked>
</p>
<p>
<select disabled name="clientes">
<option>d</option>
<option>d</option>
<option>d</option>
<option>f</option>
</select>
</p>
</form>
</body>
</html>