A method of protecting a microcomputer system against manipulation of data stored in a memory arrangement of the microcomputer system, in particular a control program stored there. Checking mechanisms are executed at preselectable points in times to check for manipulation of the data. To permit effective blocking of manipulated data stored in the memory arrangement, new data is stored at least partially in a volatile memory, in particular in a random access memory, in reprogramming or new programming of the memory arrangement, the checking mechanisms are executed and the portion of the new data stored in the volatile memory is copied to the memory arrangement if no manipulation of the new data has been detected.