The effect of various representative hæmotoxins on human and rabbit red blood cellsin vitrowas studied. It was found that as a result of the action ofB.welchiitoxins produced from a variety of strains of the organism a definite anisocytosis was produced and that the change in size of the cells followed a regular sequence. The first change to be noted was a development of cells having an average diameter less than the normal (microcyte stage). Further action of the toxin resulted in the replacement of these microcytes by cells having an average diameter greater than the normal (macrocyte stage). Following this macrocyte stage it was found that there was a return of the cells to a diameter very closely approximating the normal. This change in the size of the cells did not appear as a result of the action of any of the other hæmotoxins worked with viz.,B.tetani,V.septique,Strepto.scarlatinœ,Staph.aureus.