C. parvum and BCG produced significant changes in NK, K and T cell lymphocytotoxicity using a Chang liver target cell. A consistent temporal pattern over 28 days of early depression, recovery, overshoot and then decline was described. This was particularly marked for C. parvum and 'K' cell activity. Skin test reactivity to recall antigens at 28 days was not appreciably different from the pre-immunization reactivity. The importance of using lymphocyte concentration-cytotoxicity titration curves and the linearization of such curves is discussed. An immunotherapy schedule with 3 weekly immunization intervals is proposed as the optimum schedule in patients receiving C. parvum at a dose of 2.0 mg/m2 i.v.