Cis-diamine dichloroplatinum (II) (DDP) injected i.p. or i.v. at the dosage of 0.83 mg/kg to BALB/c mice on days 0, 7 and 14 elicited antibodies against proteins (RSA, BGG) and haptens (TNP, FITC) detectable by ELISA or by passive haemagglutination. Significant increase in antibody production was observed with either route of DDP application. A similar set of antibodies against antigens which do not cross-react with the immunizing proteins was observed when mice were immunized with complexes formed by an in vitro incubation of RSA and BGG with DDP. Thus, proteins modified in vivo or in vitro by DDP may stimulate the production of antibodies of unexpected specificities.