Fraction IB (FIB) from Pasteurella pestis induced immediate protection of mice concomitantly infected with plague. Moreover, the serum withdrawn from mice 1 or 2 days after treatment with FIB passively protected mice against P. pestis. Both the serum β-globulin and leukocyte concentrations increased as protection increased during this time. Subsequently the concentrations of both β-globulin and leukocytes decreased. About 6 days after treatment there was a second period of increased resistance which was accompanied by an increase in serum γ-globulin. Leukocytosis was not accompanied by increased phagocytosis. An increase in β-globulin concentration was also induced by inoculating mice with protein from Escherichia coli, but these mice were not protected against plague. Either the β-globulin contained antibodies responsible for the early protection or the early increase in β-globulin was coincidental.