Summary Several aerobic and anaerobic species of corynebacteria were examined for their ability to stimulate the lympho-reticular system of mice. Whereas active strains produced marked increases in spleen, liver and lung weight, possessed adjuvant activity, and were capable of sensitising mice to histamine, inactive strains possessed none of these attributes. Enhancement of carbon clearance was observed only in strains that possessed the other stimulatory properties but was not as great as previously observed by other workers for Corynebacterium parvum. The majority of active strains were anaerobic and possessed quantitatively similar stimulating activities. No correlation was observed between biochemical reactions of strains and lympho-reticular stimulation.