How to set UITableView cell alpha?

I have to set the cells alpha to minimum. I try the following code in cell for row, it's not working. I've also set opaque but it's still not working. Anyone have an idea about this?

UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
if (cell == nil) 
{
    cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier] autorelease];
}
cell.alpha =0.5;