<form action="index.php" method="post" >
<tr>
<td>материал</td>
<td>цена</td>
<td><input type="text" disabled="disabled" name="gramm" size="2" maxlength="2" /></td>
<td><input type="checkbox" name="material" onchange="chageBox(this)" value="0" /></td>
</tr>
<input type="submit" value="далее" />
</form>
function changeBox(ele){
ele.form.gramm.disabled=(!ele.checked);
}