SUMMARY. Twelve selected strains of were compared quantitatively for their ability to produce heat-labile toxin (HLT); all proved to be active producers, with only a three-fold range between the highest and the lowest. Bordet-Gengou agar, charcoal agar, modified Hornibrook medium and Stainer and Schölte (12G) medium differed little in their ability to support toxin production by three strains. However, cells grown on the solid media for 24 h were slightly more toxic than their counterparts grown for 72 h whereas in the liquid media the opposite was true. The concentration of iron in the medium did not influence HLT production, but high concentrations of nicotinic acid significantly reduced the HLT content of the cells. Crude preparations of toxin underwent only a 10% loss of toxicity at—20°C and were stable for up to 2 weeks at 4°C. At 37°C, toxicity was lost within a few days. The toxin was partially purified by a series of mild procedures and had a mol. wt by gel filtration of 89 000 ±10%. HLT was toxoided by treatment with formaldehyde to give a product which was immunogenic in rabbits but not in mice. Because anti-HLT could be absorbed out of the rabbit antisera by treatment with intact , it was concluded that some of the HLT in the bacteria is surface-exposed even though the main part may have a cytoplasmic location.


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