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SUMMARY: Among a large number of strains of aerobic sporing bacilli only Bacillus cereus, B. mycoides and, to a lesser extent, B. anthracis were able to produce turbidity and formation of a curd in saline extract of egg-yolk. None of the other species tested, namely, B. alvei, B. alcalophilus, B. brevis, B. carotarum, B. circulans, B. coagulans, B. fusiformis, B. licheniformis, B. macerans, B. megatherium, B. pumilus, B. polymyxa, B. orpheus, B. repens and B. subtilis caused any opalescence in the yolk medium. The yolk reaction was due to the action of phospholipinase produced by the organisms. As in the case of Cl. welchii α-toxin, the yolk curd-forming activity of B. cereus and B. mycoides also was associated with haemolytic activity. The substances responsible for these activities appear to be similar, and may be identical. The yolk reaction has proved useful for the rapid identification of B. cereus, as it is more specific than any of the other distinguishing tests hitherto employed. The three positive organisms, B. cereus, B. mycoides and B. anthracis, have been considered by previous workers on the grounds of morphology and antigenic structure to be closely related.