The main theme of this thesis is the clinical evaluation of interferon. From the biology of the interferon system and animal experiments it can be expected that exogenous interferon will exert its optimum effect when used to prevent acute infections or to modulate chronic infections. Therefore, we administered interferon to patients with chronic hepatitis B virus infection (chapter 5) and to renal transplant recipients, in whom viral infections occur frequently in the first months after transplantation (chapter 6). The other studies in this thesis are directly related to the problems we met in the clinical studies. We wanted to study interferon in an animal renal transplantation model. For us the most obvious choice was the rat. However, little was known about the production and characterization of rat interferon. Chapter 2 describes our experiences with rat interferon. While we were well underway with the study in renal transplant recipients, we were contacted by Martin Hirsch, who was conducting a similar trial in Boston. Some of his patients receiving 3 x 106 U HLI every other day showed severe bone marrow depression. We had no such problem in our trial, but we used another type of interferon: HFI. For this reason we started a study on the t'oxicity of interferons for bone marrow in vitro.