Diethyldithiocarbamate (DDTC) has been shown to protect against the toxicity of cis-diamminedichloroplatinum(II) (DDP) without inhibition of antitumor effect. We report here that DDTC is unreactive toward DDP complexes in which both chlorides have been replaced by guanine residues but removes platinum from a variety of other ligands, and that this difference in reactivity may provide the basis for the selective protection observed with DDTC. Platinum-DNA complexes were unreactive toward DDTC (10 mM, greater than 4 h) when the platinum:base ratio r less than 0.02. DDTC did not react with the 1:2 complex of DDP:guanosine but reacted rapidly with the 1:1 complex and with the 1:2 complexes of DDP:adenosine. Reaction of DDP with DDTC was second order with a rate constant k = 4.4 M-1 min-1 at 37 degrees C, corresponding to a t 1/2 = 150 min at [DDTC] = 1 mM. Treatment of L1210 cells with DDTC (0.5-1 mM) after exposure to DDP indicated that DDTC had no effect on cell kill if DDTC treatment was delayed for 1 h after DDP. The effect of DDTC on DDP-induced DNA interstrand cross-links was also examined in L1210 cells. Interstrand cross-links were decreased by approximately 50% when cells were treated with DDTC immediately after DDP; no change in DNA interstrand cross-links was observed when DDTC treatment occurred 3 h after DDP. A modified alkaline elution procedure was used to evaluate the effects of high concentrations of DDTC, thiourea, and cyanide on platinum:DNA cross-links from L1210 DNA. Exposure to DDTC (0.5 M, 4 h) did not alter interstrand cross-links, but both thiourea and cyanide caused extensive reversal of cross-links at concentrations as low as 10 and 1 mM, respectively. Both commercial and rat kidney brush border preparations of gamma-glutamyl transpeptidase were inhibited by exposure to 2 mM DDP; exposure of the inhibited enzyme to DDTC (1 or 10 mM) resulted in significant restoration of enzyme activity. These data indicate that DDTC has unique chemical specificity in its reactions with platinum complexes and that this specificity is ideal for application as a chemoprotective drug against cis-platinum toxicity.