Heparinized whole blood, heavily parasitized with Anaplasma marginale, was collected from 3 splenectomized oxen. Buffered lactose peptone (BLP) was added to equal volumes as a stabilizer and the mixture lyophilized in 2 ml aliquots after rapid freezing. The dried material was reconstituted with 2 ml sterile water and inoculated without delay. The product remained infective for at least 6 months when stored in an ordinary household deep-freeze unit.