The reproducibility of an immunohistochemical method for visualization and quantitation of the cytotoxic drug cisplatin (CDDP) in its active position, bound to nuclear DNA, was investigated in tissues from CDDP-treated nude mice. Kidney, liver, and tumor sections were stained with a PAP technique using an antiserum elicited against CDDP-DNA adducts. The resulting brown nuclear precipitate was quantitated using a CAS 200 image analyzer. The variance components of the errors in the staining procedure and in the image analysis were estimated. The method was found to be feasible for comparisons between slides of the same type of tissue, stained in the same batch, and measured by one observer on one occasion. The method should be a valuable tool for studies of CDDP pharmacodynamics, sensitivity prediction, and the effects of interactions between CDDP and other drugs and chemomodulators.