Rabbit antisera elicited against calf thymus DNA modified cis- and trans-diaminedichlorplatinum II (DDP) [Pt/nucleotide ratio - 0.1] contain antibodies specific for Pt-modified DNA immunogen. A specific antibody against DNA-cisDDP and DNA-transDDP was obtained by affinity chromatography. In the ELISA, 3.10(-1 6) mol Pt per micrograms DNA was determined. The epitope structure was determined by using DNA modified with cisDDP and transDDP.