The presence of host antibodies against Rickettsia typhi in the hemolymph of Xenopsylla cheopis fed on immune rats was studied using an indirect immunofluorescent antibody (IFA) test and enzyme-linked immunosorbent assay (ELISA). The time course experiment revealed that antibody to R. typhi appears in the flea hemolymph after 18 hr exposure to host and persists for 24 hr. IFA tests utilizing specific antisera to rat IgG (Fc) and rat IgG (Fab) fragments, indicate that some unaltered rat IgG to R. typhi were present in the hemolymph of immune fed fleas. Host antibody to R. typhi was detected on the surface of the rickettsiae in the flea hemolymph and gut contents by IFA. Maintenance of infected fleas on immune host had no significant effect on the establishment and subsequent growth of rickettsiae within the fleas. However, 19–22 day infected fleas maintained on immune hosts failed to transmit R. typhi to baby rats.