Interferons are antiviral glycoproteins, which are secreted as a result of a viral infection and have wide-ranging antitumor and immunomodulatory effects. They have attracted much interest as immunotherapeutic agents. Interferons bind to cell surface receptors and activate secondary intracellular changes which inhibit viral replication. They can be divided into three groups: alpha (α), beta (β) and gamma (γ) interferons. All three interferons have been genetically enginered, and recombinant IFB-α,-β, and -γ are available, but IFN-α is the best studied. IFN-α is the treatment of choice for hepatitis B and C; when given systemically, it produces significant clearing of hepatitis B in chronic carriers. IFN-α has some side effects, mainly flu-like symptoms such as fever, malaise, and anorexia- all symptoms that can be tolerated. More severe effects are reversible: bone marrow depression, liver dysfunction and cardiotoxicity.