Splenic cell migration was evaluated by lymphocyte counts in splenic afferent and efferent blood, the lymphocytes being divided into groups with different mitochondrial content (corresponding to the different size of the lymphocytes). The effect of a single intraperitoneal injection of sheep red blood cells (SRBC) was studied 3–15 days after the injection. Cell counts of splenic arterial blood showed a decreased percentage of lymphocytes with low mitochondrial content after injection with SRBC. Per mm3 of blood the absolute number of lymphocytes with 0–5 mitochondria (the smallest lymphocytes) was reduced up to 40 per cent 9–15 days after injection of SRBC. The number of lymphocytes with higher mitochondrial content tended to decrease earlier, and to reach a normal, or above normal, level within a few days. The normal increase in the total number of lymphocytes in growing guinea-pigs was not affected by the treatment. No effect of SRBC on splenic cell emigration was observed for certain, although there was a possible increase 6–12 days after injection, reaching a maximum on the 6th day. Thymic weight was reduced 3 days after SRBC injection and remained somewhat lower than in control animals 9–15 days after the injection. There was no significant effect on splenic or lymph-node weight. Initially, the normal increase in cervical lymph-node weight was to some extent suppressed in the treated animals, however, and splenic weight tended to increase after SRBC.