Immunosuppressive agents were used to determine the relative importance of T and B lymphocytes in conferring protection to mice vaccinated with a live gal E mutant of Salmonella typhimurium, strain G30D. Lymphocyte transformation and serum agglutination tests showed that while cyclophosphamide (CPA) suppressed B lymphocytes, antilymphocyte sea (ALS) suppressed both T and B cells. The humoral response of vaccinated animals treated with ALS was therefore supplemented by the i.v. injection of serum from untreated vaccinated mice. CPA-treated mice could not control multiplication of the vaccinal strain which eventually killed them. There was little multiplication of the vaccinal strain in the controls and ALS-treated mice, all of which survived to challenge. The vaccinated controls and vaccinated ALS treated groups each survived infection with the challenge strain which was gradually eliminated. It was concluded that humoral immunity was of greater importance than cellular immunity in mice vaccinated i.p. with strain G30D.