Five mouse ascitic fluids (MAFs) containing monoclonal antibody to Rickettsia rickettsii were produced from three original fusions by murine hybridoma technology. The five MAFs were fractionated and purified; each contained monoclonal antibody of the immunoglobulin G2a subclass. Each monoclonal antibody-containing MAF was titrated by indirect immunofluorescence against three R. rickettsii isolates from humans and four other spotted fever group rickettsiae. Each MAF was also titrated in the complement fixation, latex agglutination, microagglutination, and indirect hemagglutination tests. Two of the MAFs were examined for their ability to prevent fever and rickettsemia in susceptible guinea pigs after a 1:100 dilution of each was mixed with viable R. rickettsii, and all five MAFs were titrated in the mouse toxicity phenomenon assay. All MAFs had high indirect immunofluorescence titers to the three strains of R. rickettsii (1:200,000 to 1:800,000), reduced indirect immunofluorescence titers to R. montana, and were nonreactive with R. akari, R. sibirica, and R. conorii. Each MAF was able to fix complement in the presence of spotted fever group antigen reagent and agglutinate a suspension of purified R. rickettsii, and each was negative in both the latex agglutination and the indirect hemagglutination tests. The two MAFs which were tested proved to be capable of preventing rickettsemia and death in guinea pigs, and each MAF was able to prevent death in mice at dilutions ranging from 1:40 to 1:80.