Hola
FNX_NET
¿Algo así...?
Código PHP:
<form>
<select onchange = "lista2.disabled=false">
<option>---</option>
<option>Uno</option>
</select>
<select name="lista2" disabled="disabled" onchange = "lista3.disabled=false">
<option>---</option>
<option>Dos</option>
</select>
</form>
Saludos,
