The authors report a case of chronic lymphocytic leukemia with thrombocytopenia caused by antiplatelet antibodies. The customary treatments of this condition having proved ineffective, therapy with large I.V. doses of Ig (0.4 g/kg body weight daily for five consecutive days) was started, which is believed to increase platelet count although the mechanism of this action is not yet fully understood. Interesting changes in laboratory parameters were observed, especially those concerning electroimmunophoresis and immunofixation and these might be useful for a better understanding of the various hypotheses that have been put forward to account for the mechanism(s) of action possibly at work. Treatment proved effective wherefore the authors hope that further studies will be carried out in order to increase our knowledge of the modulation and regulation of the immune response, a primary element in the management of these disorders which is at present only experimental.