A series of platinum dichloroethylenediamine complexes [PtCl2(R-en)] bearing a side chain on one carbon atom of the ethylenediamine ligand, with or without a functional group on the side chain, have been prepared and investigated for antitumor activity against L1210 leukemia. They were tested both in vitro, with cisplatin-sensitive and resistant cell lines, and in vivo, with cisplatin-sensitive and resistant tumors grafted i.p. in B6D2F1 mice. The rationale for this study was to test how charge, polarity and shape of the R side chain influence antitumor activity. Complexes carrying one or more ammonium groups on the side chain were all inactive. Derivatives with a carbamate function attached by the nitrogen atom, via a methylene group, to the ethylenediamine moiety ('N-bound' carbamate) were highly active in vitro and in vivo. The best results were obtained with these carbamates bearing hydrophobic substituents of intermediate size. Replacement of N-bound by O-bound carbamate or by urea groups led to decreased in vivo activity. Sulfonamide derivatives were all inactive. Good to excellent activities were also recorded for complexes bearing bulky bicycloalkyl substituents, without any functional group, attached to one ethylenediamine carbon atom. Thus, it is the steric features of the side chain rather than its polarity that appear to favor the antitumor activity of the complex. Compared to cisplatin and oxaliplatin, the present complexes do not exhibit advantages in terms of experimental antitumor activities in solid tumor models.