The affinity of ristocetin B for analogues of the C-terminal tripeptide sequence of bacterial cell wall mucopeptide precursors resembles that of vancomycin. Complex-formation requires a d-configuration in the two amino acid residues of the C-terminal dipeptide, an l-configuration is preferred in the preceding amino acid residue and positive charges on the peptide molecule decrease its affinity. The specificity of ristocetin B, however, differs from that of vancomycin in the requirements for the size of the side chains on the C-terminal dipeptide. These differences may explain the observed differences in antibiotic behaviour of vancomycin and ristocetin with particular micro-organisms. The optical rotatory dispersion and u.v.-absorption characteristics of the ristocetins are very different from those of vancomycin but nearly identical with those of ristomycin A. Aglycones prepared from ristomycin A were antibiotically active and also combined with a specific peptide.