Properdin was determined in germfree and conventional rats by the method of Pillemer et al. (Jour. Exptl. Med., 103 1, 1956). The properdin titers of germfree animals were originally significantly lower than those of the conventional animals. After exposure of the germfree animals to conventional environment the titers rose within 42 days to about the same value as that of the conventional animals. It was concluded that the properdin level like the gamma-globulin level is influenced by the normal bacterial flora.