The present work describes the discovery of an organoleptic alteration in some batchs of suckling mouse brain rabies vaccine for human use, produced in the Central Laboratory of Public Health of the Province of Buenos Aires. This alteration consists in the appearance of fibrous particles, light-white, slightly perceptible, suspended in the diluent. The production procedure is that of Fuenzalida and Palacios (4), with the modifications proposed by the Panamerican Zoonoses Center (Larghi and col.) (6), which include a refrigerated centrifugation of the viral suspension to 17,000 g during 10 minutes. This alteration caused 20% or more of dosis loss and a decrease in vaccine potency (2). In order to determine the origin of this alteration, we controlled the homogenization systems, the elements employed, the production of experimental batchs with diversity of isotonizators (dextrose-sucrose) and preservatives, the utilization of glass filters in the treatment of water, all drugs employed, variation of pH of diluent solutions, double centrifugation, the glass employed (ampoules, flasks), pumps of dispensation and wash systems (3) (5). We conclude that the alterations are produced by partial freezing of the infected brain suspension to 10% in water, during the last part of the centrifugation. Particles are formed by irreversible precipitation of a proteic nature (1) (7). It is recommended that during the production of this vaccine the temperature be maintained between 3 to 5 degrees C carefully avoiding freezing the brain suspension in distilled water.