During recent years the therapeutic indications of monoclonal antibodies are on the increase. Most monoclonal antibodies are immunosuppressants. Thus, therapeutic successes are accompanied by an increase of serious infections. Closest cooperation between the haematologist/oncologist and the infectious diseases specialist is a prerequisite for the successful outcome of treatment of the individual patient. Rituximab and alemtuzumab especially pave the way for a variety of opportunistic infections. Prophylactic drugs directed against bacterial infections and invasive fungal infections have not been proven to be useful. Although the efficacy of prophylactic cotrimoxazole against Pneumocystis carinii pneumonia has not been thoroughly examined in this setting, it is usually given at a dose of two double-strength tablets every other day. Whether reactivation of herpes simplex and varizella zoster can be influenced prophylactically has not been studied in randomised trials. In case of fever and detection of cytomegalovirus reactivation preemptive therapy should be given.