Feeding of the shrimp, Penaeus monodon, with diets containing leaf meal of the leguminous shrub, Leucaena leucocephala, resulted in complete chromatin decondensation of hepatopancreas cells. The fibrillar component of the nucleolus was decondensed in parallel, whereas the granular component remained intact. This unique combination of nuclear signs was accompanied by only moderate alterations of other cell organelles. Our findings therefore demonstrate an encouraging possibility to manipulate the chromatin organization in living cells. Furthermore, ultrastructural features obtained thus far only in isolated and chromatin-depleted nuclei could be verified. These are, for instance, filament bundles which attach the nucleolus to the nuclear periphery, or a filamentous skeleton of the nuclear pores. In addition, the attachment of chromatin to the inner membrane of the nuclear envelope was observed. Decondensation was probably caused by the major Leucaena ingredient mimosine and is obviously related to its copper chelating properties.