Peripheral blood lymphocytes from 24 patients were cultured in vitro in the presence of autologous peripheral serum and bone marrow blood serum, with and without PHA stimulation. Bone marrow serum showed a well defined effect, in the absence of PHA stimulation, on lymphocytes from 4 patients (2 affected by acute leukaemia, 1 by megaloblastic anemia and 1 by benign idiopathic paraproteinaemia), and a less defined effect on lymphocytes from 2 donors (1 affected by iron deficiency anemia and 1 by thalassemia minor). No difference attributable to the source of blood serum was observed in PHA stimulated cultures. These results do not allow defined statements. Peripheral blood lymphocytes cultured in vitro were affected by bone marrow blood serum only in some subjects. A clear cut correlation between this effect and the clinical features of our patients was not evident.