The effect of normal saline (NS) on the antitumor activity, toxicity and pharmacokinetic of cisplatin (DDP) was investigated in BDF1 mice bearing P388 leukemia. Tumor-bearing mice received 8 or 16 mg/Kg of DDP dissolved in NS or distilled water (DW) intraperitoneally. Control animals were treated with DW or NS alone. The administration of 8 mg/Kg of DDP+NS produced a significantly better survival (P less than 0.05) compared to that observed in mice receiving DDP+DW. The proportion of long-term survivors was 3.5 times higher in the DDP+NS group (39%) compared to the DDP+DW group (11%). The administration of 16 mg/kg DDP+DW was highly toxic, resulting in early deaths (MST = 5 days) and no long-term survivors. NS protected from DDP toxicity without further improving the survival achieved following the injection of 8 mg/kg DDP+NS. Investigation of platinum pharmacokinetics showed that NS significantly decreases both plasma and tissue concentrations of total platinum, mainly through a decrease in the amount of platinum bound to high molecular weight plasma proteins. HPLC studies indicated that mice receiving 8 mg/kg DDP+NS or DDP+DW fail to show clear differences both in the total ultrafilterable platinum and unchanged DDP in plasma ultrafiltrate. Conversely, mice treated with DDP+NS had higher concentrations of platinum-species in plasma ultrafiltrate than mice receiving DDP+DW. These latter results, together with the observation that NS decreases the amount of platinum bound to plasma proteins, suggest that the effect of NS does not solely depend in vivo on the ability of the chloride ion concentration to stabilize the DDP molecule and suppress the formation of DDP metabolites, but also on its ability to prevent DDP toxicity by reducing the protein binding of DDP aquated products.