It has been demonstrated that media derived from cultures of human monocytes can stimulate the IgM and IgG anti-sheep erythrocyte response of normal or T cell-deficient murine splenocytes in vitro. This culture fluid also causes a slight stimulation in the rate of B cell division and in the number of antibody-secreting cells occurring in the absence of added antigen. The active factors, which are released from the monocytes during the first 2 days of culture, appear to affect the murine B cells throughout the culture period.