The interferons (IFNs) were identified as novel, endogenous antiviral agents in 1957. Shortly thereafter, antiproliferative and immunostimulatory activities were identified for these compounds. Based on these observations, partially purified IFNs entered clinical trials in the 1970s and recombinant IFNs in 1980. IFNs have demonstrated important clinical activity in hairy cell leukemia, melanoma, renal cell carcinoma, and Kaposi's sarcoma as monotherapy. Shortly after their introduction into clinical trials, however, preclinical studies demonstrated synergistic interactions between IFNs and cytotoxic drugs. Numerous preclinical trials have demonstrated a broad spectrum of interactions between IFNs and at least 20 cytotoxic agents both in vitro and in vivo. Early clinical trials suggest a benefit to combinations of fluorouracil and recombinant interferon alfa in refractory gastrointestinal malignancies. Combinations of IFNs and cytotoxic agents deserve further investigations; however, different principles apply for combining IFNs with cytotoxic drugs than for the design of combination chemotherapy regimens.