The last two decades have seen some significant advances made in the recognition of infection problems in the immunocompromised host and in their prevention in a rapidly expanding population. Many areas urgently await much-needed improvements, particularly anti-bacterial decontamination and fungal and viral prophylaxis. Despite interesting pilot studies there has been a failure to fully evaluate potential strategies in properly designed trials. Now is the time for large studies which control for the multitudinous variables of patient population such as disease status, protective environment, degree of diet sterility, and types of GI and mucocutaneous decontamination. Meanwhile, it is impossible to make hard and fast rules for prophylaxis for all circumstances. Clearly, most of the measures which we have discussed are applicable only where there is profound immunosuppression. Practice should be based on a careful evaluation of the local flora and fauna. Table 6 details an outline of the Royal Free prophylaxis schedule concurrently used during the treatment of acute non-lymphoblastic leukaemia and following marrow transplantation. This type of protocol is an attempt at short-term 'total' decontamination which appears justifiable in this very high-risk group where the invading organisms are a greater immediate risk than the disease under treatment. Our hope is that the scientific foundations for such regimens will rest on firmer ground in the future.