Conglutinin is a mammalian C‐lype lectin which agglutinates iC3b‐coated erylhrocytes, Ingram [13] found that euglobulin from bovine serum may confer partial protection against experimental infections in mice. We now present evidence that the protective activity in euglobulin against infections of BALB/c mice with Salmonella typhimurium is mediated by conglutinin. Conglutinin also demonstrated antibacterial activity against E. coli and S. typhimurium in vitro. The expression of this activity required the presence of heat‐labile serum factors and peritoneal exudate or spleen cells, but not antibodies to the bacteria. Antibacterial activity was also demonstrated when the bacteria were prelreated with serum at 37°C before incubation with conglutinin and cells. The activity of conglutinin was not observed when factor I‐deficient or EDTA‐treated serum was used instead of normal serum. The active peritoneal exudate or spleen cells showed adherence to plastic.