Monoclonal antibodies which bind to tumour cell surface antigens have produced regressions of malignancies in an increasing number of clinical trials. The largest experience to date is in the treatment of refractory B and T lymphoid tumours using a variety of intravenously administered mouse monoclonal antibodies. Treatment with antibodies against common differentiation antigens or very specific anti-idiotype antibodies has been effective in both cases. Toxicity has been acceptably low. A number of problems which limit the application and efficacy of monoclonal antibody therapy of lymphoid malignancy have been identified. Most prominent among these are tumour heterogeneity, which allows non-antibody binding subpopulations of the tumour to escape therapy, and the patient's immunological response to the monoclonal antibody-tumour cell complex. As more experience is accumulated, solutions to these problems will be found.