Developing on QT 4.2.3

I try to set an item of a table not editable:

QTableWidget *mytable;
QTableWidgetItem myvalue1;
myvalue1 = mytable->item(0,1);

2 ways

if i try:
myvalue1->setFlags(myvalue1->flags() | Qt::ItemIsEditable);
doesen't work.

if i try
// myvalue1->setFlags(Qt::ItemIsEditable);
this works.

Why this ???

Second Problem is that i want to unset the flag not editable,
but if i try
myvalue1->setFlags(myvalue1->flags() & ~Qt::ItemIsEditable);

This doesen't work.