Chick embryo fibroblasts cultivated in vitro and treated with cis-dichlorodiamino platinum (II) have been analysed by cytological and cytochemical methods. Morphological alterations (in the nucleoli, in the chromatin and in the cytoplasm), inhibition of DNA and RNA syntheses and of cell multiplication have been noted. Under some conditions, cells are blocked just before entering into mitosis (G2 block) and can become polyploid. A high degree of cell degeneration has also been observed.