Spectrophotometric studies have been undertaken of the interaction of various iron-based systems with the anti-tumour agent, 2,3-dihydro-1H-imidazo(1,2-b)pyrazole (IMPY, NSC (51143), a ribonucleotide reductase inhibitor. No evidence was obtained of direct complexation in aqueous media at 25 degrees C between IMPY and Fe2+ (aq) (pH 1.5-6.8) or Fe3+ (aq) (pH 1.0-3.5), nor with a mu-oxo-bridged iron dimer (Fe--O--Fe) system. There was definitive spectral evidence of complexation of IMPY with protoporphyrin IX iron (II) at pH 7.4 and 12.9 both in the absence and presence of carbon monoxide bound at the haem-iron site. Binding of IMPY to protoporphyrin IX iron (III), in contrast, was not detected. Binding between IMPY and various iron sites important in biochemistry is discussed briefly, especially in relation to the structural properties of IMPY (from X-ray data) and the Fe--O--Fe bridge system in ribonucleotide reductase and model systems. The difficulties of the use of free heterocyclic nitrogenous bases in medicinal chemistry are discussed.