Carrageenan, a sulphated polysaccharide, and rabbit anti-mouse macrophage serum, were used to inhibit macrophage function in BALB/c mice as well as to deplete macrophages from spleen cell cultures in an attempt to determine the requirement for macrophages in the immune response to several thymus-independent antigens. Carrageenan inhibited macrophage function and was cytotoxic at low concentrations. The ability of T and B lymphocytes to undergo mitogen-induced proliferation in the presence of PHA and PLS, respectively, was not affected by in vitro exposure of lymphoid cells to carrageenan. BALB/c mice injected with carrageenan demonstrated a suppressed immune response to SRBC, a thymus-dependent antigen, but not to E. coli LPS, polyvinyl-pyrrolidone or dextran B-1355S, all of which are known to be thymus independent antigens. The sensitivity of the in vivo immune response to SRBC after depletion of macrophages by carrageenan treatment was confirmed in vitro using the Marbrook--Diener culture system. The in vitro immune response to TNP-LPS was unaffected by either carrageenan treatment or treatment of BALB/c spleen cells with AMS and complement. The results of experiments which utilized the two anti-macrophage reagents, carrageenan and AMS, both in vivo and in vitro systems, suggest that the immune response to thymus-independent antigens does not require the participation of macrophages.