Código:
<form id="form1" name="form1" method="post" action=""> <label> <select name="select" id="select"> <optiononchange="document.getElementById('text1').disabled=true;">A</option> <option onclick="document.getElementById('text1').disabled=false;">B</option> </select> </label> <label> <input type="text" name="text1" id="text1" /> </label> <form>