We have performed Phase I trials of two synthetic double-stranded polyribonucleotide complexes--poly(I,C)-LC, a complex of polyinosinic-polycytidylic acid with poly-L-lysine and carboxymethylcellulose, and poly(I,C)-L, which lacks carboxymethylcellulose--in patients with advanced cancer. With poly(I,C)-LC, several treatment schedules were investigated in an attempt to decrease toxicity and maximize interferon (IFN) induction. The best tolerated was an alternate-day schedule, with gradual dose escalation. Daily short infusions and continuous (24-h) infusions were tolerated less well. Maximum tolerated doses varied over a several hundredfold dose range. Toxicity consisted of fever, rigors, hypotension, and blood count depression. Two patients treated with poly(I,C)-L developed systemic allergic reactions, and antibodies to poly(I,C)-L and its components were detected in the serum of some patients treated with both compounds. IFN-alpha was induced in most patients at serum levels similar to those achieved after intramuscular administration of human IFN-alpha. Of 32 patients, one with renal cell carcinoma showed partial tumor regression. Poly(I,C) complexes are effective IFN inducers in humans, but their toxicity limits their use in cancer patients.