This study investigated the effect of splenectomy on the production of ablastin and the elimination of Trypanosoma lewisi from the circulation of infected Porton rats. Rats were splenectomized or sham-splenectomized either before or at various stages during the infection and the course of the parasitaemia compared with untreated control animals. Ablastin titres in the serum were measured by a sensitive in vitro assay and were found to be the same in all the rats, regardless of the nature or timing of the operation. Splenectomized animals eliminated the parasite more slowly from the circulation than either the sham-splenectomized animals or unoperated controls. It is concluded that the presence of the spleen enables the rat to clear the parasite more rapidly from the circulation, but that the spleen is not essential for the production ablastin during the infection.