In the present study we investigated the role of monocytes and of their soluble products (prostaglandins and hydrogen peroxide) in the modulation of the immune response in 50 untreated patients with Hodgkin's disease (HD) compared with a group of healthy donors. The primary response in vitro has been studied with the method of haemolytic colonies in soft agar. A defective in vitro antibody production has been observed in HD patients. Both Indomethacin addition (10(-6) M, final concentration) and depletion of plastic adherent cells, slightly increased the number of haemolytic areas in cultures from HD patients as compared with healthy donors. Similarly, the addition of catalase (8000 U/ml) which destroys H2O2, that is the main mediator of monocytes suppressor activity in normal subjects, did not restore the response of peripheral blood mononuclear cells (PBMC) from HD patients. These results suggest that monocytic cells play a minor role, if any, in the depression of the immune response in HD patients.