Twenty-four patients with cancer and concomitant infections with either herpes virus hominis or varicella zoster virus were treated with polyinosinic-polycytidylic acid (rIn.rCn) to determine: 1) the reliability of rIn.rCn to induce interferon production, and 2) the toxicity of the drug. Seven additional patients with herpes zoster were observed as controls. Two lots of rIn.rCn were used; Lot 1 was consistently effective in stimulating serum interferon at doses of 9 and 12 mg/kg, whereas Lot 2 was effective at doses of 3-12 mg/kg. There was no correlation between rIn.rCn doses within these ranges and the resultant interferon levels. Generally, peak serum interferon occurred within the first day. Toxicity to rIn.rCn consisted of fever in 21/24 patients, mild elevation of liver enzymes in 8/24 patients, and laboratory abnormalities of coagulation in 9 patients. The coagulation abnormalities appeared linearly related to the dose of rIn.rCn used. All these abnormalities were reversible, and none were considered severe of life-threatening. Since rIn.rCn was effective in stimulating interferon and since toxicity was considered acceptable, a randomized double-blind study was initiated to determine whether rIn.rCn is effective in the treatment of herpes zoster.