A macrophage aggregation test which uses peritoneal exudate cell suspensions (PEC) in tubes in the presence of antigen was found to correlate with cutaneous delayed hypersensitivity. Moreover, the supernatant fluids from lymphoid cell cultures from antigen sensitive animals caused aggregation of nonsensitive PEC in the presence of antigen. Thus, antigen leads to the release of a macrophage aggregation factor (MAF) from sensitive lymphoid cells in culture. By serial dilution, the MAF titers were determined for the supernatant fluids of various lymphoid cell cultures.