Using ABAP instrution to change screen field properties

Hi,

I've a screen (SE51) and i need to change field properties using an ABAP instrution.

I need to do this... if variable LV_NDEP_IN is not initial, user can't change field content.

I've tried this...

IF lv_ndep_in IS INITIAL.

SET CURSOR FIELD 'LV_NDEP_IN'.

ELSEIF lv_ndep_in IS NOT INITIAL.

screen-input = ' '.

MODIFY SCREEN.

ENDIF.

but doesn't work.

Can somebody help me ?

Thanks,