cis-Diamminedichloroplatinum(II) (cis-DDP) has a broad clinical application as an effective anticancer drug. However, development of resistance to the cytotoxic effects is a limiting factor. In an attempt to understand the mechanism of resistance, we have employed a host cell reactivation assay of DNA repair using a cis-DDP-damaged plasmid vector. The efficiency of DNA repair was assayed by measuring the activity of an enzyme coded for by the plasmid vector. The plasmid expression vector pRSVcat contains the bacterial gene coding for chloramphenicol acetyltransferase (CAT) in a configuration which permits expression in mammalian cells. The plasmid was transfected into repair-proficient and -deficient Chinese hamster ovary cells, and CAT activity was subsequently measured in cell lysates. In the repair-deficient cells, one cis-DDP adduct per cat gene was sufficient to eliminate expression. An equivalent inhibition of CAT expression in the repair-proficient cells did not occur until about 8 times the amount of damage was introduced into the plasmid. These results implicate DNA intrastrand cross-links as the lesions responsible for the inhibition of CAT expression. This assay was used to investigate the potential role of DNA repair in mediating cis-DDP resistance in murine leukemia L1210 cells. The parent cell line L1210/0 resembled repair-deficient cells in that about one adduct per cat gene eliminated expression. In three resistant L1210 cell lines, 3-6-fold higher levels of damage were required to produce an equivalent inhibition. This did not correlate with the degree of resistance as these cells varied from 10- to 100-fold resistant.(ABSTRACT TRUNCATED AT 250 WORDS)