Summary. Six patients with autoimmune thrombocytopenic purpura (three of whom had CLL/NHL and one previous Hodgkin's disease) refractory to conventional therapy were treated with an antilymphocyte monoclonal antibody directed at CDw52 (CAMPATH-1). We observed response in four of the five evaluable patients; in three patients the response has lasted more than 4–9 months. However. response did not occur in most cases until 4–6 weeks after the commencement of the antibody therapy, suggesting that the mechanism of action is probably that of immune modulation.