A modified medium for the cultivation of bacteria-free ciliates and flagellates has been described. Immune sera against various species of Protista have been produced by injecting the living organisms into laboratory animals. These sera contain thermostable immobilizing antibodies. The cross-immobilization and the cross-absorptive technics have been applied to the flagellates. Colpidium campylum, Colpidium striatum, and Glaucoma pyriformis have again been shown to be very closely-related antigenically. Khawkinea halli and Khawkinea ocellata have been shown to be antigenically different. Serological tests have indicated that the Astasias, Euglenas, and Khawkineas are closely related. The taxonomic significance of these findings has been discussed.