The humoral antibody response and the number of pronephric antibody-secreting cells were examined in naturally Bothriocephalus acheilognathi-infected carp. Cyprinus carpio, and in those injected intraperitoneally with an extract of the cestode. In the extract-injected fish, specific antibody was detected 3 weeks after a second injection given 2 weeks after the primary injection, and antibody levels persisted for more than 200 days. A third injection also enhanced the antibody level in the extract-injected carp. The numbers of antibody-secreting cells were significantly higher in carp injected 3 times with the extract than in the control. In naturally-infected fish, the serum antibody levels and the number of pronephric antibody-secreting cells were higher in infected fish than in uninfected individuals although this difference was not statistically significant. The relevance of these results to immune protection against infection is discussed.