Human lymphoid tissues and peripheral blood leucocytes and monocytes were studies with respect to the synthesis of complement components (C1q, C3 and C4) using an in vitro culture technique. All of the lymphoid tissues investigated (bone marrow, thymus, lymph node, spleen, tonsil, adenoid) synthesize complement components in different patterns. C3 was produced by all lymphoid tissues except the spleen, which was the only lymphoid tissue in which C4 production was regularly found. C1q synthesis was demonstrated in the spleen and adenoid cultures, and occasionally also in those of lymph node tissue. Lymphocytes in peripheral blood from normal individuals and in thoracic duct lymph, and also from patients suffering from chronic lymphatic leukaemia, do not synthesize any of these complement components. Peripheral blood leucocyte samples from normal individuals, containing 60 per cent lymphocytes and 40 per cent monocytes, do synthesize C3, however. Separation of the monocytes from these samples showed that it was in these cells that the synthesis of C3 occurred. Production of C3 by mononuclear phagocytes is also supported by the finding that peripheral blood leucocytes from patients suffering from acute monocytic leukaemia synthesize C3. C1q and C4 synthesis could not be demonstrated in any of the cultures of circulating leucocytes.