Atmega1280: ADC-Interrupt-Vector is not called

Hello,

during first experiments with an Atmega1280 (powered with 5V at 16Mhz) I 
encountered one problem with the ADC-interrupt, although I followed the 
specification :
The interrupt vector is not called although I enabled ADC functionality, 
it's interrupt and the global interrupt, additionally I set the 
ADSC-flag to
initiate the conversion, register settings:

DIDR0 = 0xFF ;    -> digital inputs disabled
DIDR2 = 0xFF ;    -> digital inputs disabled
ADCSRA = 0x8F;    -> ADEN, ADIE , prescaler 128, ADSC set at conversion 
start
ADCSRB = 0 ;
ADMUX = 0 ;       -> AREF used

Did I miss any specification detail ?
It would be very helpful if someone could give me a hint what might be 
the cause for this.