Abstract: The present invention is directed to the field of animal and human health, and more particularly to pharmacological uses of analogs or mutants of interferon-tau (IFN-&tgr;) that differ from native IFN-&tgr; because of substitutions of amino acids near the amino terminus of the IFN-&tgr; molecule that impart improved biological activity. The IFN-&tgr; mutants described in this disclosure have low toxicity, retain the same or slightly reduced antiviral activity compared with highly effective IFN-alpha, and have enhanced antiproliferative activity compared to native IFN-tau, making them useful in treating viral infections, cancer, and immune system diseases including autoimmune diseases. The present invention is also directed to a method for making novel recombinant proteins, especially interferons, interleukins, and cytokines, polypeptide hormones and other biopharmaceuticals that have improved biological activity over known proteins and/or lower toxicity and/or increased stability.