Stability constants of complexes formed by nickel(II) and zinc(II) ions with DL-penicillamine and L-cysteine have been calculated from pH titration data at 20° and I= 0·1 (NaCIO4) by use of a computer-based method. With penicillamine, the nickel(II) complexes are simple mononuclear species of the types ML and ML22–, whereas with zinc(II) protonated species are also present. Both metals form polynuclear and protonated complexes with cysteine. Copper(II) ion is reduced by these ligands.