We have previously reported that the production of the second complement component (C2) by human monocytes can be stimulated by co-culture with lymphocytes and antigen or with lymphokine-rich culture supernates. Using lymphocyte subpopulations separated on the basis of rosetting with sheep erythrocytes (T-lymphocytes), we now report that only cultures composed of antigen-stimulated monocytes and T lymphocytes could produce this lymphokine activity. Macrophage migration inhibitory factor (M.I.F.) was also present in this supernate. Although antigen-stimulated B lymphocytes (rosette-negative lymphocytes), B lymphocytes with monocytes, monocytes only, and T lymphocytes only could not produce this lymphokine activity, supernate of antigen-stimulated B lymphocytes and monocytes did contain M.I.F., indicating that molecules with this activity are not necessarily identical with those responsible for stimulating C2 synthesis by monocytes.
Further, Sephadex G-100 gel filtration of supernates from antigen-stimulated lymphocyte cultures resulted in fractions also capable of stimulating monocytes to increase their C2 production.