1, Approximately 90% of all salmonella strains tested, other than S.galllnarun and S.pullorun were found to produce typical haemagglutinating fimbriae. The remaining 10% of strains were permanently non-fiabrinte. S.galljjqarun and S,pullorun produced non-haemagglutinating fimbriae, morphologically identical with those of other salmonellae. 2. Antisera were prepared against the fimbriae of 25 strains of 22 serotypes. In the course of studies on the antigenic structure of the fimbriae of Salmonella, by a cross-absorption technique, five different fimbria antigens were discovered - Fl, F2, F3, F4, F5. Antigen F1 was present in all (97) fimbriate strains tested of 60 serotypes, including S.g&llinarum and S.pullorum. Antigen F2 occurred in only 26 serotypes, Antigens P3» P4 ana P5 were produced by only 4, 3 and 5 of the serotypes respectively. A provisional table of fimbrial antigenic formulae of the salmonella group is presented in Table 21, All fimbriate strains of the same serotype were found to be identical in fimbria antigenic structure. Tne importance of these findings in the preparation of diagnostic sera and suspensions is disoussed. 3. Experiments were performed in mice and chickens to investigate the effect of fimbriae on the virulence of salmonellae. Organisms in the fimbriate phase were found to be more virulent than the equivalent non-fimbriate organisms for mice by oral inoculation, but not intraperitoneally. It was suggested that in some way the fimbriae overcame the protective effect of the intestinal commensals, since both phases were of equal virulence when given orally to mice with streptomycin-sterilised intestinal tracts, or to 1-day-old chickens. Evidence was also obtained suggesting that fimbriate organisms penetrated the intestinal mucosa to reach the blood and internal organs more readily than the non-fimbriates, and persisted for longer periods in the intestine. 4. Sera and dead vaccines immunising against the fimbrial antigen only were found to produce & significant protection against oral infection of mice with a fimbriate strain of S.typhlmuriuta, almost equal to that conferred by the corresponding; O-serura or vaccine. All living vaccines tested showed a very high degree of protection against S.typhlmurirua - no conclusions oould therefore be drawn regarding the effect of fimbriae.