Summary: Staphylococcic toxoid was precipitated with 0.5 to 7.0 per cent potassium alum added in 10 per cent aqueous solution. The maximum yield was obtained over the range 1.5 to 3 per cent alum. Greater purification, as measured by nitrogen content, was obtained with either more or less alum, but the yield was sharply reduced. The yield was greater when the precipitation was carried out at 20°C. than at 4 to 8°C. and when the mixture was stirred constantly. No loss in purity was found with these modifications. The antigenic potency of alum-precipitated staphylococcic toxoid and of the solution of the precipitate in rochelle salt was tested on rabbits and horses. As a primary stimulus, the alum preparations were definitely better than the unmodified toxoid, the alum-tartrate solution being intermediate in value. As a secondary stimulus, no significant difference was observed. The superiority of the alum preparations would appear to lie mainly in the much earlier response obtained. This was particularly well shown in the hyperimmunization of two horses. The toxic properties of certain toxoid preparations were not removed by alum-precipitation.