Equilibrium study on the mixed-ligand complex formation of M2+ ions (M = Co, Ni, Cu, and Zn) with α-d-(-)aminobenzyl penicillin popularly known as ampicillin, (hereafter, ampH±), and nucleic bases, viz. adenine, guanine, thymine, uracil, and cytosine (B), in aqueous solution at 37°C at a fixed ionic strength, I = 0.1 M (NaNO3), indicates the formation of complexes of the types: M(amp), M(B), M(amp)(B), M(H-1 amp)(B), and M(H-1amp)(B)(OH). Stability constants of the ternary complexes are in the order: Co2+ < Ni2+ < Cu2+>Zn2+ with regard to the metal ions and guanine>adenine>uracil>thymine>cytosine with regard to the nucleic bases. Complex formation equilibria have been worked out on the basis of species distribution curves. © 1995.