A comparative study has been made of rabbit precipitating and coprecipitating anti-egg albumin antibody in rabbits repeatedly injected with soluble and particulate antigens. Four different antigens were used. a) Soluble egg albumin. b) Polymerized egg albumin. c) Egg albumin linked Brucella abortus 19 strain. d) Brucella abortus 19 strain. Rabbits were subcutaneously injected every 2 weeks during 29 weeks. Before each inoculation they were bled and the concentration of serum precipitating and coprecipitating antibodies was determined. In animals inoculated with the soluble antigen, coprecipitating antibodies were present during the whole course of the immune response and they constituted 10% of the total antibody population. In rabbits injected with particulate antigen (egg albumin linked Brucella abortus) coprecipitating antibodies were also present during the whole course of the immune response but their serum concentration was not the same in different periods of time. An increase in coprecipitating antibodies began at the 13th week reaching 50% of the total antibodies at the 28th week. Similar results had been obtained in rabbits injected with Brucella s. p. In animals inoculated with polymerized egg albumin, coprecipitating antibodies increased to high levels from the 1st to the 23rd week; during this period it constituted 50% of the total antibody population, after which, the animals became tolerogenic. On the basis of these results, the increase in coprecipitating antibodies in rabbit sera would be related to: 1) antigen physicochemical characteristics (soluble o particulate); 2) the number of epitopes in the particle, and 3) the size of the antigen particles.