1.The serum of 5 patients and 1 guinea pig after recovery from leptospiral jaundice (Weil's disease) gave positive Pfeiffer reactions and protected guinea pigs against L. icterohaemorrhagiae and L. icteroides.
2.A series of titrations showed that these 6 immune sera were essentially equal in their effects against these 2 strains of leptospira.
3.From an etiological standpoint L. icteroides is a synonym of L. icterohaemorrhagiae.
4.If the serum of a patient convalescent from an attack of acute infectious jaundice gives positive Pfeiffer reactions with protection of guinea pigs against either L. icterohaemorrhagiae or L. icteroides, the diagnosis of leptospiral jaundice (Weil's disease) is justified. Negative reactions are consistent with the diagnosis of yellow fever.