SUMMARYThe course of the reaction produced by intracutaneous injection of the toxin ofLyngbya majusculaGomont is that of a severe acute inflammatory reaction. No protective or sensitizing effect is induced by a previous exposure. The response from intravenous injection into a rat indicates that the toxin acts as a general cell toxin. The protozoanTetrahymena pyriformisand rabbit erythrocytes are lysed by the toxic principle, which also possesses antibacterial activity. Of the organisms tested,Mycobacteriumspecies are markedly inhibited, whileBacillus cereus, Gaffkya tetragena, andSarcina luteaare slightly to moderately inhibited. Other components of the alga, which are steam distillable, have been found to have antibacterial activity but are not involved in the skin reaction.