Salmonella typhimurium C5 is under normal conditions (physiological saline containing 0.002 M Mg2+) resistant to the action of antibody and complement (C). It becomes sensitive, however, when suspended in tris(hydroxymethyl)-aminomethane buffer (Reynolds and Pruul, 1971; Reynolds and Rowley, 1969). The interaction of complement components with this strain sensitized with specific antibody has been studied to identify the intermediate step at which inhibition occurs. The components C1 yields C2 react normally, as has been shown by lysis of complement-treated cells incubated with complement in ethylenediaminetetraacetic acid. Also, the reaction of C3 can be demonstrated by positive immune adherence and agglutination with anti-C3. The complement-treated cells do not, however, react with rabbit C6 to 9 or rabbit serum lacking C6 in tris(hydroxymethyl)aminomethan buffer. We conclude from these date that C5 can not react effectively under normal conditions. In contrast, if bacteria-antibody complexes are pretreated with rabbit serum lacking C6 in tris(hydroxymethyl)aminomethane buffer, they are readily lysed by incubation with C6 to 9. Thus, C5 can react with the bacterial surface in tris(hydroxymethyl)-aminomethane buffer.