Changes in antigen molecules that resulted in enhanced or decreased uptake by the phagocytes resulted in higher or lower immune responses, respectively. After the development of methodologies for obtaining live exudate cells rich in phagocytes and for pulsing these with antigen, the immune response to macrophage-associated antigens was possible to assay, using combinations of in vivo and in vitro methods. The presentation of antigen bound to live macrophages to the lymphocytes was a highly efficient mode of generating an immune response. In the marine system, the requirement for phagocytes or other acc|essory cells was such that as little as 1% contamination with phagocytes still enabled a T-cell proliferative response to develop. Thus, depletion procedures had to be extremely efficient. Macrophages are definitely involved in some response to many of the conventional polyclonal stimuli. Involvement of the macrophage may be by way of secreted active molecule. Fc fragments of Ig act as a polyclonal stimulant only after a processing of the fragment by the macrophages. The developments in macrophage biology over the past few years were not predicted till 1972.