SummaryA study has been made of the distribution of the heat labile protective antigen ofSalmonella typhinuriumamongst other strains of apparently unrelated gram‐negative bacteria. Evidence is presented to show that this antigen is possessed by a number of different species of bacteria. Results obtained on the protective effect of various antigens fromSalmonella typhimurumsuggest that only certain antibodies are effective in promoting killing by phagocytic cells, though all may’ enhance phagocytosis.