The cestodes Echinococcus granulosus, Taenia multiceps, T. pisiformis, T. hydatigena, Hymenolepis diminuta, Moniezia expansa and Anoplocephala perfoliata all produced substances that stimulated thymidine incorporation by whole blood lymphocyte cultures. This mitogenic activity was demonstrated in metacestode cyst fluids, live protoscoleces and scoleces, parasite culture supernatants, and extracts of adult parasites. T. multiceps metacestode cyst fluid mitogen adhered to, but would not pass through, cellulose dialysis tubing. This adherence was reduced or prevented by D-glucose and by proteins. The mitogen was weakly anionic. With Sephadex G75 gel filtration, its elution volume was greatly decreased when the elution buffer contained 0·5 M D-glucose. Mitogenic activity was protease resistant and could be separated by gel filtration from all the cyst fluid proteins.