SUMMARY. Modulation of was induced by growth in Hornibrook medium with a high content of magnesium sulphate (C-medium); four pathophysiological activities in the cells or in the whole culture were measured at intervals. Modulation, shown by the extensive loss of protective antigen, histamine-sensitising factor, leuko-cytosis-promoting factor, heat-labile toxin and X-mode specific envelope proteins, occurred during the first 10 h of incubation of X-mode cells in C-medium at 35°C and before the onset of the logarithmic phase of growth. The rapidity of these losses was greater than could be explained by a simple growth-dilution effect and did not appear to be due to release of activity into the culture fluid. It seemed, therefore, that there was selective destruction of pathophysiological activities as well as cessation of synthesis. The activities appeared to be lost at different rates. Mouse-protective activity was lost more rapidly than histamine-sensitising and leukocytosis-promotmg activities; heat-labile toxicity was lost more slowly or less completely. Modulation was shown to be easily reversed.


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