Supernatant fluids from murine spleen cell cultures incubated with concanavalin A for 48 hr contain a factor(s), soluble immune response suppressor (SIRS), which suppresses plaque forming cell responses to sheep erythrocytes by murine spleen cells in vitro. In the present studies, some of the biochemical and biophysical properties of SIRS were investigated. SIRS was non dialysable; the suppressive activity was stable at 56°C for 30 min, but was destroyed by treatment at 70°C for 30 min, 80°C for 10 min, or at pH 2. The suppressive activity was not absorbed by the stimulating antigen, SRBC, or antisera against murine IgG or μ chain, suggesting that SIRS does not contain immunoglobulin determinants. Murine spleen and thymus, but not kidney cells, however, absorbed SIRS activity. Enzyme treatments revealed that SIRS was resistant to DNase and RNase, but was destroyed by trypsin and chymotrypsin. In gel filtration with Sephadex G 100, SIRS activity eluted in the fraction corresponding to m.w. in the range between 48,000 and 67,000. With polyacrylamide gel electrophoresis, SIRS activity migrated in the region cathodal to albumin. Isopycnic centrifugation in a cesium chloride gradient suggested that SIRS is a glycoprotein. These supernatant fluids with SIRS activity were also found to contain macrophage migration inhibitory factor (MIF). In the experiments using gel filtration, polyacrylamide gel electrophoresis, and isopycnic centrifugation to fractionate supernatant fluids, SIRS and MIF activity were found in the same fractions, and to date we have been unable to dissociate definitively SIRS activity from MIF activity.