In the present paper we present data showing that the effect of the binding of the antitumour drug Pt-pentamidine to nucleosomal DNA is the opposite to that of the cis-DDP compound since it causes strong stabilization of the double helix to heat denaturation and because in nucleosome:Pt-pentamidine complexes the nucleosomal denatured DNA is able to reassociate at 71°C. Upon binding, the pentamidine ligand, by itself, also produces stabilization of the nucleosomal DNA but the effect is lower than that induced by Pt-pentamidine. It seems that in Pt-pentamidine:nucleosome complexes about 50% of the adducts are formed during the first hour of incubation of the nucleosomes with the drug since the increase in Tm of the DNA of these complexes is 53% of the total increase in Tm of the DNA of the Pt-pentamidine:nucleosome complexes formed in 48 h. The ‘in vitro’ screening of the antiproliferative activity of Pt-pentamidine against 60 tumour cell lines indicated that this antitumour compound shows higher antiproliferative activity against small cell lung, non-small cell lung and melanoma cancer lines than against the rest of the cell lines.