SUMMARY Two types of subunit, with molecular weights estimated to be 28000 and 8000, were demonstrated in the cholera exo-enterotoxin by sodium dodecylsulphate polyacrylamide electrophoresis. The light (L) component but not the heavy (H) was demonstrable in the antigenically identical natural toxoid. A split of the H component by reduction and alkylation suggested it to be a disulphide-bridged dimer. Gel filtration at acid pH permitted separation of the H and L subunits in isolated form. Fractions with isolated subunits were non-toxic (skin test), in contrast to fractions with intermediate filtration rate containing both subunits. Fractions with a L: H ratio of about 3.5 were even more toxic than the intact toxin (L: H ratio about 2). Addition of the isolated subunits to toxic fractions generally caused inhibition and only exceptionally potentiation of toxicity. The data suggest that the cholera toxin might consist of an aggregate of seven L and one H subunits while the natural toxoid seems composed of only the aggregated L chains.