Suspensions of magnetite albumin microspheres (MAM), a new biodegradable particulate iron superparamagnetic MR contrast material, were evaluated in vitro and in vivo as an oral contrast agent. MAM is stable over a broad range of pH and tolerates proteolytic enzyme exposure over 24 hr in vitro. MAM possesses a much larger magnetic moment than do paramagnetic contrast agents. The transverse relaxation rate (R2) of MAM can be as much as 40 times the longitudinal relaxation rate (R1). In vitro spectroscopy studies confirm the potency of MAM in promoting T2 relaxation at concentrations of 10-1000 mg/l. Preliminary studies in rabbits and dogs show that in contrast to oral gadolinium-DTPA, which causes increased signal in bowel, MAM causes marked signal loss in the stomach and small bowel on both T1- and T2-weighted pulse sequences. Radionuclide labeling studies of MAM suspension with 99mTc show no evidence of absorption of MAM suspension from the gastrointestinal tract in small animals. Superparamagnetic suspensions such as MAM that reduce bowel signal on T1- and T2-weighted pulse sequences offer the unique benefit of reducing motion artifacts throughout the gastrointestinal tract, which should allow for improved evaluation of intra- and retroperitoneal diseases, particularly with high-field strength and gradient-echo "fast-scan techniques." Unlike paramagnetic material, MAM appears effective as a small-bowel contrast material.