Protective immunity of carp to ichthyophthiriasis has been confirmed, and demonstrated for the first time in juvenile carp, 10-12 weeks old. Standard immunisation procedures were developed here using the theront in preference to cysts. Immunisation included exposure of fish on 3 separate occasions of 14 day intervals to doses of approximately 2,000 theronts per fish, 80/ cm³. Procedures were controlled so that infections were not allowed to continue beyond the primary stage pH (7.0-7.2 ) and temperature (20±2° C) were maintained throughout experimental periods. Four weeks after third immunising dose, fish were exposed to a potentially lethal challenge, approximately 8,000 theronts per fish, 320/cm³. Following immunisation , fish showed total protection up to 1 month and decreasing protection up to 3 months during which period, mortalities were recorded on challenge. Humoral antibody was monitored at specific stages of experimental infections, peak response 6-8 weeks following exposure with detectable levels of antibody persisting for at least 12 weeks. Immunosuppression was demonstrated following intraperitoneal administration of synthetic corticosteroid Triamcinolone acetonide, doses of 200 µg, 100 µg and 10 µg gˉ¹ body weight, and corticosteroid Hydrocortisone 21-hemisuccinate, doses of 100 µg and 10 µg gˉ¹ body weight, given 14 days after challenge. Immunosuppression was not associated with any significant fall in antibody titre. Studies in cross immunity between Tetrahymena pyriformis (CCAP 1630/W Claff, 1939 (w)) and Ichthyophthirius multifiliis showed no evidence of the former conferring protection to ichthyophthiriasis. Methods of administration of T. pyriformis to juvenile carp included intraperitoneal injection of freeze dried cilia and whole, live T. pyriformis. The kinetics of the humoral response were measured over 12 weeks, peak antibody titres occurring 6-8 weeks following antigen administration. Proliferative responses measured by autoradiography were recorded prior to peak antibody production. Overall results are discussed in relation to immunosuppression, mechanisms of immunity and control and treatment of the disease.