Cultivated animal cells (mouse peritoneal macrophages, chick fibroblasts or mouse Ehrlich tumor cells) or Zea mays root tips are treated with cis-Pt (II). Various effects (chromatin dispersion or condensation, pycnosis) are observed under some experimental conditions in all cell types. Cytoplasmic ribosomes in helical aggregates appear but only in vegetal cells. Mitosis and cell cycle disturbances due to cis-Pt (II) are probably related to chromatin alterations. We suggest that the latter and helical polyribosomes are produced by cis-Pt (II) reaction with nucleic acids in these structures.