Preparations of mucopeptide were solubilized from formamide-extracted residues of groups A and B streptococcal cell walls by sonication and purified by column chromatog. on Sephadex. Other preparations were made by digestion of particulate mucopeptide with lysozyme and Streptomyces albus muramidase. Anal. of these materials showed chem. similarity and a range of mol. size estimated to be from >20 × 106 to <5 × 103. The larger components reacted serol. as precipitinogens and the smaller as haptenic inhibitors. Mucopeptide preparations of larger mol. size produced migration inhibition reactions (MIR) with peritoneal cells from nonsensitized rats and guinea pigs. In general, MIR and dermonecrotic reactions decreased with decreasing mol. size. Antimucopeptide antibody only slightly reduced mucopeptide-induced MIR. Isolation of a migration inhibition factor by Sephadex chromatog. from supernatants of cell cultures incubated with mucopeptide was unsuccessful. Stimulation of cells with Bayol F was required to produce cells susceptible to MIR. Under the exptl. conditions for demonstration of MIR, mucopeptide did not directly cause lysis or reduce viability of the peritoneal cells used in the test system.