Abstract: As cercarial secretion material (CSM) from the acetabular glands is the first subs-tance to contact the host during infection, the question is raised whether the CSM is immunogenic. We have compared the immunizing abilities of newly-transformed schistosomula-like organisms with intact CSM and with schistosomula in which the CSM has been depleted by prolonged cultivation. A Puerto Rican strain of Schistosoma mansoni was used. Female CF1 mice were immunized by the injection of schistosomula which had been transformed from X-irradiated cercariae at 36kR. CSM-intact schistosomula-like organisms were used for immunization immediately after their transformation by the syringe technique. CSM-depleted schistosomula were prepared by incubating them in Earlc's saline with 0.65% lactabumin hydrolysate in a CO2 incubator at 37℃ for 14 hrs. Purpurin and PAS staining showed that both the pre- and post-ace tabular glands were depleted of CSM in the cultured, but not in the uncultured schistosomula. For each immunization, 500 schistosomula-like organisms or schistosomula were injected intramuscularly into the right hind limb. Mice were immunized 3 times at 4 week intervals, then challenge-infected with 100 cercariae 30 days after the last immunization. Age-matched unimmunized control mice were similarly exposed to 100 challenge cercariae. Worms were recovered by perfusion 41-44 days after challenge.Fifty-three mice were divided into three groups. GroupⅠ consisted of 18 unimmunized controls; Group Ⅱ, 20 mice immunized with CSM-intact schistosomula-like organisms; and Group Ⅲ, 15 mice immunized with CSM-depleted schistosomula. Mean worm recoveries from the challenge infections were 50.5 (±13.8), 21.9 (±11.0), and 28.9 (±8.7) for Groups Ⅰ,Ⅱand Ⅲ, respectively. Analysis of variance on arcsin transformed data indicated significant differences among the three group means (F= 32.82, P0.001). Duncan's test showed significant differences among all three means at the 5% level. Worm reductions were 56.6 and 42.8% in Groups Ⅱ and Ⅲ, respectively. The results of the present study show that CSM is a protective immunogenic substance in this case, but it should be considered only as an auxiliary immunogen, because CSM-depleted schistosomula were also protective, though to a less extent.