Patients with Burkitt's lymphoma have impaired delayed hypersensitivity to recall antigens (mumps, streptokinase‐streptodornase candidin and PPD) in the presence of a tumour load. The degree of impairment at presentation (which may reach total anergia) is dependent upon the clinical stage and correlates significantly with survival rate (p < 0.02), sustained remission rate (p < 0.05) and remission duration (p < 0.01). Cutaneous reactivity 3 months after chemotherapy is significantly improved overall in spite of the potentially immunosuppressant effects of cyclophosphamide but immuno‐suppression recurs in patients who relapse. BCG, applied by dermal scarification resulted, after 4 weeks, in significant improvement of cutaneous reactivity (p < 0.001), but no improvement in prognosis. The most probable explanation of these findings is that the impairment in delayed hypersensitivity correlates with the tumour volume, which in turn governs prognosis. The level of general immunological responsiveness at presentation seems unlikely to be a crucial factor per se in governing the response to chemotherapy.