SUMMARY Tris and EDTA combined to form a homologue which has far greater chelating ability than EDTA alone. The weaker serine and NH,Cl homo- logues of EDTA replaced EDTA+tris in causing toxicity and permitting lysozyme lysis in Azotobacter vinelandii. Serine could effectively substitute for tris in these systems for Pseudomonas aeruginosa. These compounds had relatively little effect on Escherichia coli. The toxicity of EDTA+tris for these organisms could be alleviated by pre-incubation in physiological saline (0.15 M-NaCl). Subsequent lysozyme lysis was prevented by this treatment. The removal of metals from A. vinelandii cysts by EDTA+tris was also inhibited. The EDTA + tris + metallo chelate complexes were inactivated (dissociated) by saline.