The underlying basis for hypogammaglobulinaemia in patients with chronic lymphatic leukaemia (CLL) was investigated by measurement if immunoglobulin produced in vitro in cultures of pokeweek mitogen-stimulated B and T lymphocytes. B and T cells were separated by sheep red blood cell rosette techniques and, by culture of these cells from CLL patients in various combinations with B or T cells from normal subjects, it was possible to measure independently the function of B lymphocytes and the helper or suppressor function of T lymphocytes. By these methods it was found that the B lymphocytes of six of eight patients failed to produce immunoglobulins in vitro. B lymphocytes from two patients appeared to produce immunoglobulins in vitro. T lymphocytes from five of the eight patients had low or undetectable helper T cell function and in six patients their T lymphocytes had excessive suppressor activity in comparison to T lymphocyte populations from normal subjects. Whether the primary abnormality in the CLL T cell populations was a deficiency of helper T cells or excess of suppressor T cells was uncertain from these studies. These results suggest that immunoglobulin production by B lymphocytes from most patients with CLL was abnormal but also that T cells from CLL patients may be abnormal in respect to their role in immunoglobulin production at an early stage of the disease. These findings may assist in understanding the pathogenesis of this disease and lead to new approaches in treatment.