Summary When isolated, the meningococcus strain under study was not agglutinated in the standard monovalent rabbit serums of groups I, II and III; nor in polyvalent serums obtained by immunization of horses with 4, 6, 20 and 60 strains. After prolonged cultivations, however, it acquired a certain degree of agglutinability in polyvalent serum and a slight tendency to react in low dilutions of monovalent serums of group II and finally of group III. It was found to be highly agglutinable in the homologous monovalent serums obtained by immunization of rabbits, a horse and a mule after it had been grown on media for about a year. Although the organism should be classified in the indeterminate group “X” of the international classification, its remote relationship to groups I, II and III is shown by the definite agglutinative action of its homologous monovalent serums with representative cultures of these groups. It absorbed the group agglutinins from these serums as well as its own specific agglutinins. No specific agglutinins for strains of the classified groups I, II and III were, however, absorbed by it from the corresponding monovalent serums. The observations recorded in this study, as far as they relate to the classification of the meningococcus, are in accord with the results published in a previous paper as to the existence of three distinct groups. The remaining atypical strains should be placed in group “X” of the international classification. Finally, this strain is a striking example of a meningococcus giving rise to a serious infection that apparently yielded to treatment with polyvalent serum in which, when first isolated and for some time afterward, it failed to agglutinate. Moreover, it is interesting to note that as the culture became more agglutinable it reacted at least as well with the polyvalent serums produced by immunization with six selected strains belonging to the three main groups of meningococci, in which groups the “Rna” strain could not be classified, as with the polyvalent serum produced against sixty strains among which were included many atypical strains.