Summary. The present study investigated the nature of ablastin, a factor present in the serum of rats infected with Trypanosoma lewisi and which inhibits the parasites' division. Ablastin was unstable to dialysis at pH 18, was not adsorbed from serum by trypanosomes and could not be induced in vivo by means other than a natural infection. Attempts to purify ablastin from serum by conventional chromatographic techniques were unsuccessful. Removal of over 90% of the immunoglobulins from ablastinic serum did not reduce the ablastin titre. It is concluded that ablastin is unlikely to be an immunoglobulin as has been previously suggested.

Summary. This study investigated the effect of splenectomy on the production of ablastin and the elimination of Trypanosoma lc\vi.s-i from the circulation of infected Porton rats. Rats were .splenectomized or sham-splenectomized either before or at various stages during the infection and ihe 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 shamsplenectomized 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 spteen h not essential for the production ablastin during the infection.