The in vitro activity of cercariae of Trichobilharzia elvae, T. physellae, and T. stagnicolae was observed in serums of rabbits and ducklings with homologous and heterologous infections, and compared with the effects of serums from the uninfected host species. Inactivated (56 C for 30 minutes) serums from the uninfected hosts produced no detrimental effect on the cercariae in 24 hours at room temperature. Low dilutions of rabbit antiserum caused a characteristic pericercarial envelope formation (CHR). In more dilute rabbit antiserum and low dilutions of duckling antiserum only a partial envelope formed and a mucoid secretion appeared at the anterior end. In higher dilutions of the antiserums of both hosts, the cercariae did not form recognizable envelopes but did secrete large amounts of material from the oral openings. It is suggested that the phenomena of envelope-formation and agglutination arise from different reacting principles. In strong antiserum, the envelope forms immediately and, as has been pointed out by others, seems to have its origin in the fine extra-cuticular material present on the cercariae. In weaker antiserums, the envelope does not form over the anterior end of the cercariae. In these the oral secretion, which appears to be sticky, seems to be responsible for the tendency of the cercariae to adhere to the slide and to one another. In the weakest of the reacting antiserums, no envelope is seen to form but profuse oral secretion occurs and becomes dispersed in the surrounding medium. This is apparently responsible for the firm agglutination of the cercariae.