The effector mechanism of the protection against re-infection with eggs of Taenia taeniaeformis was evaluated in rats by analysing protection dynamics against an ongoing primary infection. Immune spleen cells were prepared from F344 donor rats 10 days after oral inoculation with 200 eggs and used for adoptive transfer experiments. When F344 recipient rats were injected intravenously with immune spleen cells (2 x 10(8)) from 2 to 48 h after a primary egg inoculation, there was a statistically significant reduction in the number of metacestodes recovered at day 30 of the infection. The highest protection figures were obtained in rats given immune cells at 12 or 24 h of the primary infection. The maximum reduction induced with immune cells was 92% when they were injected at 12 h of the ongoing infection. It is strongly suggested that the vulnerable stage of the parasite damaged by immune spleen cells is the initial stage of infection (oncosphere and post-oncosphere stages in the liver within 2 days, especially between 12 and 24 h).