Microfilariae in whole uncentrifuged cat blood were transfused from infected to normal cats. The recipient cats were bled frequently and the life-span of the microfilariae in the peripheral circulation was determined; The life-span on microfilariae in different cats was ver variable. Microfilariae were detected fro 2-136 days after inoculation. In immunized cats microfilariae could not be detected 18 hours after inoculation. Any attempt to concentrate or clean the microfilariae caused them to disappear from the circulation more quickly.