The effects of antinuclear antibodies on nuclear material can be observed directly in free leucocyte nuclei in the supravital preparation according to Engel. 2. The antinuclear antibodies prevent the lysis of free nuclei, they obstruct the normal degeneration of nuclei. 3. The nuclear blockade is recognizable in the dark-clodded, rigid nuclei which remain small. These blocked nuclei are the visible expression of a development of complexes between chromatin and antinuclear antibodies. 4. It is supposed that the protective effect of antinuclear antibodies consists of the obstruction of the DNases and histon-proteases bound to chromatin. 5. The effects of antinuclear antibodies is in agreement with the structure of the nucleosomes of chromatin. 6. The nuclear blockade caused a persistence of antigenic material, which can create an increase of antibody production. This means that in vivo the antinuclear blockade is an effect which sustains illness.