Interferons are currently the most widely used biological response modifiers. They are of high clinical value in haematological malignancies (chronic myelogenous leukaemia, multiple myeloma, non-Hodgkin lymphoma), in solid tumours (malignant melanoma, hypernephroma, pancreas neoplasms, carcinoid tumours, Kaposi\'s sarcoma, glioma, in ovarium, cervix and bladder carcinoma, and in basalioma) and in infectious diseases (chronic hepatitis B, chronic non-A/non-B hepatitis, chronic delta hepatitis, AIDS, Papova virus and Rhinovirus infections, leishmaniasis, leprosy) and some other conditions. Although the mechanism of action of interferons has not been explained in every detail these agents are promising therapeutic means in a number of diseases.