Blood was cultured from 17 normal subjects, 28 cases of untreated chronic lymphocytic leukaemia (CLL) and 41 cases of Hodgkin's disease. Macrophages were not observed in PHA-stimulated cultures of normal subjects, of CLL or of 18 cases of Hodgkin's disease. The latter had an almost normal rate of lymphocyte blast transformation (70,2 +/- 7,2%).on the other hand, macrophages were numerous in PHA-stimulated cultures of 23 cases of Hodgkin's disease with a low blast transformation (28.4 +/- 13,7%). In Hodgkin's disease the development of macrophages in PHA cultures may be related to the lower blast transformation and cytotoxic activity of lymphocytes. In CLL the dilution of lymphocytes and monocytes by leukaemic B cells may account both for PHA unresponsiveness and low yield of macrophages in culture.