The effect of Fe3+ ions on the hydrolysis of benzylpenicillin in aqueous solution below and above the critical micellar concentration (c.m.c. = 0.26 M) has been studied. The catalytic effect of Fe3+ ions on hydrolysis of this drug has been demonstrated; the reaction rate constant, calculated from UV absorption data of non-micellar solutions, decreases with concentration. It is likely that Fe3+ ions form complexes with the products of degradation. The bright brown slightly soluble product formed in micellar solutions was analyzed by UV - Vis reflection, IR and mass spectroscopy and, in agreement with the results of elemental analysis, is postulated to be an octahedral binuclear complex, Fe2BP4(OH)2. This study has been linked with the clinical observation that prolonged treatment with penicillin leads to anemia.