We have demonstrated that autologous macrophages enhance the growth of human tumor cells in soft agar. In an attempt to identify the macrophage subpopulation responsible for enhancing tumor growth, we depleted adherent cells of macrophages bearing surface Ia antigens, and assessed the ability of the residual cell population to support tumor growth. Adherent cells constituted 18-36% of cells from 20 effusions tested. A variable proportion of these adherent cells (33-60%) were lysed by anti-Ia-like antibody and complement. These was no correlation between cloning efficiencies of cells in the original cell suspension and the percentage of Ia-positive adherent cells initially present. Clonogenicity was reduced in all cases by removal of adherent cells. Adherent cells, depleted of Ia-positive macrophages, were able to enhance the growth of nonadherent tumor cells. The growth enhancement was dependent on the number of Ia-negative cells added. The number of colonies grown in the presence of Ia-negative macrophages was between 34 and 350% greater than the number of colonies observed when untreated adherent cells were added in 17 of 21 cases. The effect of the Ia antibody on the functional activity of the adherent cells was complement and dose dependent. Both heteroantibodies and monoclonal antibodies produced a similar effect. The results indicate Ia-negative macrophages could enhance the growth of human tumor cells in soft agar. In addition, Ia-positive macrophages may have suppressed the growth of clonogenic tumor cells Ia-positive macrophages may have suppressed the growth of clonogenic tumor cells.