Biodegradable particles were produced by the cross-linking of starch with epichlorohydrin. Diethylenetriaminepenta-acetic acid (DTPA) was covalently linked to the particles by using DTPA bisanhydride. The small, gadolinium-labelled particles were 40-260% more efficient in vitro proton relaxation agents than the corresponding unbound chelate gadolinium-DTPA. The relaxation properties were dependent on the metal chelate, the particle size, the metal content, and the degree of substitution (d.s.). For the small gadolinium-DTPA particles, an increased d.s. decreased the rate of degradation by alpha-amylase.