The experiments reported below followed the observation that living dysentery bacteria (S-variant) are not agglutinated after 4 hr. contact with an homologous antiserum. Somewhat later agglutination appears, but does not reach the end-titre. The next day fine granula are visible on the bottom of the test-tube which are readily dispersed by gentle shaking. This form of agglutination was designated ‘soft’. If the bacteria are heated for 30 min. at 100°C. agglutination appears within 2 hr. at all dilutions up to the end-titre of the serum. The next day a heavy sediment is visible, and in low serum dilutions a cuticula covers the bottom of the test-tubetities injected intravenously were 0·1, 0·25, 0·5 and 1·0 mg.; the latter were relatively harmless and quantities of 0·2–2·0 mg. were injected.