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The ε-toxin of Cl. welchii Type D has been isolated in an immunologically and electrophoretically homogeneous state, by elution from DEAE cellulose. Such a preparation, containing 10,000 Lf mg. N, sedimented as a single symmetrical peak in the ultracentrifuge, but still contained some non-protein N. The molecular weight of ε-toxin was estimated as 38,000 and its toxicity in mice as 280,000 MLD mg. protein N. Its combination with horse antitoxin was studied by neutralization tests in vivo, by the flocculation reaction (optimum proportions), by quantitative precipitation and by double diffusion in agar gel. The immunological and chemical relationships between the non-toxic ε-precursor and the active toxin derived from it by tryptic (or other enzymic) hydrolysis, and toxoid made from either material by treatment with β-propiolactone, were also investigated. [ABSTRACT FROM AUTHOR]